Diffraction grating-based wavelength selection unit

ABSTRACT

An optical wavelength selection apparatus containing a surface-relief transmission diffraction grating, a collimating lens for collimating a beam incident to the diffraction grating, and a focusing lens for focusing the beams diffracted by the diffraction grating. The diffraction grating, after having been subjected to a test condition of 85 degrees centigrade and a relative humidity of 85 percent for at least 500 hours, has diffraction efficiency performance within 6 percent of that achieved before being subjected to these test conditions.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

[0001] This application is a continuation-in-part of applicant'scopending patent application Ser. No. 09/761,509, filed on Jan. 16,2001.

FIELD OF THE INVENTION

[0002] A diffraction grating-based wavelength selection unit.

BACKGROUND OF THE INVENTION

[0003] U.S. Pat. No. 5,917,625 discloses, in FIG. 28, amultiplexing/demultiplexing device which utilizes a transmission typediffraction grating 140 disposed in front of a reflection mirror 115.The entire disclosure of this United States patent is herebyincorporated by reference into this specification.

[0004] A device similar to that disclosed in U.S. Pat. No. 5,917,625 isclaimed in U.S. patent application Ser. No. 09/193,289. This UnitedStates patent application is discussed in Columns 1 and 2 of U.S. Pat.No. 6,108,471, the disclosure of which is also hereby incorporated byreference into this specification.

[0005] The U.S. Ser. No. 09/193,289 patent application claimed anoptical multiplexing and demultiplexing device comprising a fibermounting assembly for securing a plurality of optical fibers,collimating and focusing lens, a transmissive grating including adiffractive element formed from a photosensitive medium, and a mirrorfor receiving at least one beam coming from at least one of theplurality of optical fibers via the lens and the grating and forreflecting one or more of the beams back through the grating and thelens to at least one of the optical fibers. The photosensitive mediumdisclosed in U.S. Ser. No. 09/193,289 was dichromate gelatin (DCG).

[0006] DCG transmissive gratings are transmission volume phase gratingsand, thus, the diffracting grating layer in these grating elements tendto have a thickness which is significantly larger than the correspondinglayer in surface-relief grating elements. Thus, such gratings tend tochange their optical properties with changes in temperature more thanthat achieved with surface-relief gratings fabricated on low thermalexpansion substrate materials. Furthermore, because of their thickness,DCG gratings tend to be more sensitive to angular alignment issues thanare surface-relief gratings; and, because of such alignment issues, theDCG gratings are not readily usable for broad spectrum wavelength beamapplications that require equal diffraction efficiency for all of thespectral components of the beam. For fiber-optic telecommunicationsystems having wavelength channel signals over the spectrum range offrom 1280 to 1620 nanamometers, the DCG grating-based devices tend notto be as useful for this system application as surface-reliefgrating-based devices.

[0007] Surface-relief reflection grating elements are well known tothose skilled in the art, and their diffracting grating layer issubstantially thinner than that incorporated in DCG grating elements;consequently, they do not suffer from many of the disadvantages of DCGgratings. The properties of surface-relief reflection grating elementsare disclosed in Christopher Palmer's “Diffraction Grating Handbook,”Fourth Edition (Richardson Grating Laboratory, Rochester, N.Y., 14605).Reference also may be had to a paper by E. G. Loewen et al. entitled“Grating efficiency theory as it applies to blazed and holographicgratings,” (Applied Optics, Volume 16, page 2711, October, 1977).

[0008] While surface-relief transmission grating elements are not aswell known or used as surface-relief reflection grating elements, theyare commercially available from Holotek LLC of Henrietta, N.Y.Surface-relief transmission grating elements have the same advantagesrelative to DCG gratings that surface-relief reflection grating elementshave; and they provide even more advantages than surface-reliefreflection grating elements when used in fiber-optic communicationdevices. In particular, they can provide higher wavelength dispersionpower while still achieving essentially equal diffraction efficiencyvalues for S and P polarized optical components. However, when thegrating surfaces of prior art surface-relief transmission gratingelements are subjected to a temperature of 85 degrees centigrade at arelative humidity of 85 percent for two hours or less, the gratingsurfaces are degraded until the grating structure disappears. This testis often referred to as the “Bellcore High Temperature High HumidityStorage Test for Fiber Optic Devices.”

[0009] It is an object of this invention to provide a surface-relieftransmission grating with improved durability when subjected to theBellcore High Temperature High Humidity Storage test conditions.

[0010] It is another object of this invention to have such improvedsurface-relief transmission gratings have greater than 70 percentdiffraction efficiency values for S and P polarized optical componentswhile achieving essentially equal diffraction efficiency values forthese polarization components, that is, the S and P polarizations havediffraction efficiency values within about 5 percent of each other.

[0011] It is yet another object of this invention to have such improvedsurface-relief transmission grating use a low thermal expansionsubstrate material and, thereby achieve a change in grating line spacingthat is in an accept range when the grating is used over the 70 degreecentigrade temperature range specified for fiber-optic communicationdevices.

[0012] It is yet anther object of this invention to have such improvedsurface-relief transmission grating surface be encapsulated and, therebyprotect the grating surface from being damaged due to handling andcleaning of the grating element, as well as, from contaminants, liquidsor solvent vapors that could damage the grating surface.

[0013] It is yet another object of this invention to provide devicesincorporating such improved surface-relief transmission grating.

[0014] It is yet another object of this invention to providegrating-based devices having higher wavelength dispersion power whileproviding essentially equal radiometric throughput efficiency values forS and P polarized optical components, that is, the S and P polarizationshave device radiometric throughput efficiency values equal to withinabout 5 percent of each other.

SUMMARY OF THE INVENTION

[0015] In accordance with this invention, there is provided an opticalwavelength selection unit comprising a surface-relief transmissiondiffraction grating which, after having been subjected to a testcondition of 85 degrees centigrade and a relative humidity of 85 percentfor at least 500 hours, has diffraction efficiency performance within 6percent of that achieved before being subjected to these testconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The claimed invention will be described by reference to thespecification, and to the following drawings in which like numeralsrefer to like elements, wherein:

[0017]FIG. 1 is a partial sectional view of one preferred transmissiongrating element of the invention;

[0018]FIG. 2 is an enlarged view of a portion of the transmissiongrating element of FIG. 1;

[0019]FIG. 3 is a flow diagram illustrating one preferred process forpreparing the transmission grating element of FIG. 1;

[0020]FIG. 4 is a graph illustrating the effect of varying the ratio ofcertain grating characteristics upon grating diffraction efficiency ofthe transmission grating element of FIG. 1;

[0021]FIG. 5 is a schematic illustrating a spectrophotometer whichutilizes the transmission grating element of FIG. 1;

[0022]FIG. 6 is a schematic of a spectrophotometer which utilizes thetransmission grating element of FIG. 1;

[0023]FIG. 7 is a schematic of a dual pass grating-based wavelengthselection unit which utilizes the transmission grating element of FIG.1;

[0024]FIG. 8 is a schematic of another dual pass grating-basedwavelength selection unit utilizing the transmission grating of FIG. 1;

[0025]FIG. 9 is a schematic of yet another dual pass grating-basedwavelength section unit utilizing the transmission grating element ofFIG. 1;

[0026]FIGS. 10A and 10B are side and top views, respectively, of atransmission grating-based demultiplexer fiber-optic unit;

[0027]FIG. 10C is a schematic view of the fiber-optic input/output arrayused in the demultiplexer of FIGS. 10A and 10B;

[0028]FIGS. 11A and 11B are side and top views, respectively, of atransmission grating-based fiber-optic spectrophotometer unit;

[0029]FIG. 11C is a schematic view of the fiber-optic input/output arrayused in the spectrophotometer of FIGS. 11A and 11B;

[0030]FIG. 12 is a schematic view of a dual pass grating-basedwavelength section unit that utilizes the transmission grating of FIG.1;

[0031]FIGS. 13 and 14 each present a schematic of a spectrophotometerwhich utilizes the transmission grating of FIG. 1;

[0032]FIGS. 15, 16, 17, 18, 19, 20, 21, 22, 23 and 24 each present aschematic of a surface-relief grating-based device which utilizesphysically cascaded grating elements;

[0033]FIGS. 25A, 25B, and 26 illustrate wavelength-division add/dropmultiplexer devices that incorporate the dual cascaded grating elementof FIG. 20;

[0034]FIG. 27A is a schematic side view of a transmission grating-baseddemultiplexer fiber-optic unit;

[0035]FIG. 27B is a schematic top view of some the optical componentsused in the demultiplexer device of FIG. 27A;

[0036]FIG. 28 is a schematic of yet another dual pass grating-basedwavelength section unit utilizing the transmission grating element ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037]FIG. 1 is a sectional side view of a preferred sinusoidalsurface-relief transmission diffraction grating element 10 comprised ofa substrate 12 and a grating forming layer 14 containing surface-reliefdiffraction grating 15. This FIG. 1 illustrates the angular relationshipbetween the incident optical beam 1 and the diffracted optical beams 2,3relative to the normals 4,5 to the grating surface for this gratingelement 10. In the embodiment illustrated in FIG. 1, the incident beam 1is comprised of λ₁ and λ₂ wavelength components and makes an angle ofθ_(i) with the normal 4 to the substrate surface. After propagatingthrough the substrate 12 and grating forming layer 14, the beam 1 isincident on the surface-relief grating 15. A portion of the incidentbeam 1 intensity is undiffracted and exits the grating as the zerothorder beam 6 at an angle θ_(o) relative to the grating normal 5, whilethe remaining beam intensities for each of the wavelength components ofbeam 1 are diffracted into first order λ₁ wavelength beam 2 and firstorder λ₂ wavelength beam 3 having angles of θ_(d) and θ_(d)+Δθ_(d),respectively, with regard to the grating normal 5 for the case whereλ₂>λ₁. Because, in the embodiment depicted in FIG. 1, the gratingforming layer 14 is parallel to the substrate surface on which itresides and the substrate 12 has parallel surfaces, one does not have toinclude the index of refraction of either the substrate or gratingforming layer into the grating equation used to calculate the angularrelationship between incident and diffracted beams for the gratingelement 10. Under the parallel plate conditions depicted in FIG. 1,θ_(i) can be used as the incident angle in the grating equation and,therefore, the undiffracted zeroth order beam makes an angle ofθ_(o)=θ_(i) with regard to the normal 6 to the grating surface.

[0038] The surface-relief diffraction grating illustrated in FIG. 1 is asurface-relief transmission diffraction grating, i.e., a transparentdiffraction grating that serves to transmit light. Surface-reliefdiffraction gratings are well known and are referred to in, e.g., U.S.Pat. Nos. 6,157,042 (metal surface-relief diffraction grating with agallium arsenide substrate), 6,108,135, 5,569,904, 5,539,206, 5,363,226(surface-relief reflection diffraction grating), 5,162,929, 5,089,903,4,842,633, 4,206,295, 4,204,881, 4,289,371, 4,130,347, 4,057,326, andthe like. The entire disclosure of each of these United States patentsis hereby incorporated by reference into this specification.

[0039] Referring again to FIG. 1, and in the preferred embodimentdepicted therein, it will be seen that the substrate 12 is atransmissive material, i.e., a material with a transmittance (the ratioof the radiant power transmitted by an object to the incident radiantpower) of at least about 70 percent for the wavelength spectrum to beused with the grating element. In fiber-optic telecommunication devices,such wavelength spectrum is generally from about 1280 to about 1620nanometers.

[0040] The substrate 12 preferably is of high optical quality, i.e., itintroduces less than 0.25 wave of either spherical or cylindricalwavefront power into the transmitted beam; the term wavefront power isdiscussed in U.S. Pat. Nos. 5,457,708, 5,264,857, 5,113,706, 5,075,695,and 4,920,348, the entire disclosures of which are hereby incorporatedby reference into this specification. As will be apparent to thoseskilled in the art, this means that the preferred substrate 12 has flatsurfaces which, in one embodiment, are preferably substantially parallelto each other, that is, being parallel within about 1 arc minute of eachother.

[0041] The substrate 12 is preferably optically homogeneous, i.e., allcomponents of volume in the substrate 12 are the same in composition andoptical properties. Optically homogeneous materials are disclosed in,e.g., U.S. Pat. Nos. 6,120,839, 6,103,860, 6,084,086, 6,080,833,6,019,472, 5,970,746, 5,914,760, 5,841,572, 5,808,784, 5,754,290, andthe like; the entire disclosure of each of these United States patentsis hereby incorporated by reference into this specification.

[0042] The substrate 12 depicted in FIG. 1 consists of material whichhas a coefficient of thermal expansion of from about 2×10⁻⁵ to −1×10⁻⁶per degree centigrade. It is preferred that the coefficient of thermalexpansion to be about 6×10⁻⁷ to about −6×10⁻⁷ per degree centigrade.

[0043] One may use a variety of transmissive materials known to thoseskilled in the art. Thus, by way of illustration and not limitation, onemay use optical glass, plastics, glass-ceramic, crystalline materials,and the like. Suitable materials include, e.g., “CLEARCERAM-Z” (aglass-ceramic material made by the Ohara Incorporated of Japan), ULE (aultra-low expansion glass sold by the Corning Company of Corning, N.Y.),fused silica, BK7 optical glass, plexiglass, crystalline quartz,silicon, etc. ULE glass is made by doping fused silica with titaniumand, thus, has essentially the same optical properties as fused silica.ULE glass is referred to in, e.g., U.S. Pat. Nos. 6,048,652, 6,005,995,5,970,082, 5,965,879, 5,831,780, 5,829,445, 5,755,850, 5,408,362,5,358,776, 5,356,662, and the like; the entire disclosure of each ofthese United States patents is hereby incorporated by reference intothis specification.

[0044] The substrate 12 preferably has a refractive index of from about1.4 to about 4.0. In one embodiment, the refractive index of substrate12 is from about 1.43 to about 1.7.

[0045] Referring again to FIG. 1, and in the preferred embodimentdepicted therein, the thickness 16 of substrate 12 is generally fromabout 0.5 millimeters to about 100 millimeters and, preferably, fromabout 2 to about 20 millimeters. The thickness 18 of the grating forminglayer 14 generally ranges from about 1 micron to about 5 microns. Theratio of thickness 16 to thickness 18 is generally at least about 500/1and, more preferably, at least about 1,000/1.

[0046]FIG. 2 is an exploded partial sectional view of area 20 of FIG. 1,depicting the surface-relief diffraction grating 15 in greater detail.As will be seen from FIG. 2, in the preferred embodiment depicted thediffraction grating 15 is comprised of a base 22 integrally connected toupstanding periodically spaced grating lines 24. The periodically spacedgrating grooves 26 are disposed between adjacent grating lines 24.

[0047] In one embodiment, the surface-relief grating 15 is formed in thegrating forming layer 14 by a photographic process, such as holography.In another other embodiment, the surface-relief grating 15 is formed inthe grating forming layer 14 by replication means. In anotherembodiment, the surface-relief grating 15 is etched into the substrate12 material using either chemical or ion beam milling techniques.Normally a grating formed in a photoresist material by photographicmeans serves as the mask for these etching techniques. Fabrication ofsurface-relief diffraction gratings by holographic, replication, and ionand chemical etching techniques are described in the Erwin G. Loewen etal. book entitled “Diffraction Gratings and Applications” (MarcelDekker, Inc., New York, 1997).

[0048] In one preferred embodiment, the grating forming layer 14consists essentially of material with an index of refraction of fromabout 1.4 to about 1.8 and, more preferably, from about 1.43 to about1.55.

[0049] In the embodiment depicted in FIG. 2, the grating 15 has asubstantially sinusoidal shape. In another embodiment, not shown,grating 15 has a substantially rectangular shape (see, e.g., page 180 ofsaid Loewen book). In another embodiment, the gratings 15 may have asubstantially triangular shape (see, e.g., page 180 of said Loewenbook).

[0050] Sinusoidal diffraction gratings are disclosed in U.S. Pat. Nos.6,026,053, 5,757,544, 5,755,501, 5,742,262, 5,737,042, 5,696,628,5,341,213, 4,842,969, 4,729,640, 4,062,628, and 3,961,836; the entiredisclosure of each of these United States patents is hereby incorporatedby reference into this specification.

[0051] In one embodiment, the grating is formed from a photoresistmaterial which, after being heat-treated, becomes a substantially drysolid material. It is preferred to use a positive photoresist material.Positive photoresist materials are well known to those skilled in theart and are disclosed, e.g., in U.S. Pat. Nos. 6,094,410, 6,094,305,6,051,348, 6,027,595, 6,005,838, 5,991,078, 5,965,323, 5,936,254,5,910,864, 5,907,436, 5,838,853, and the like; the disclosure of each ofthese United States patents is hereby incorporated by reference intothis specification.

[0052] In the preferred embodiment, the positive photoresist materialused is Shipley S1813 Photo Resist (manufactured by The Shipley Companyof 455 Forest Street, Marlboro, Mass.). This positive photoresistmaterial is comprised of from 71 to 76 parts of electronic gradepropylene glycol monomethyl ether acetate, from about 10 to about 20parts of mixed cresol novolak resin, from about 0.01 to about 1 parts offluoroaliphatic polymer esters, from about 1 to about 10 parts of diazophotoactive compound, and from about 0.01 to about 0.99 parts of cresol.

[0053] The Shipley S1813 Photo Resist is believed to belong to a classof diazonaphthoquinone (DNQ)-novolak positive photoresists; see, e.g.,pages 431-511 of James R. Sheats et al.'s “Microlithography Science andTechnology (Marcel Dekker, Inc., New York, 1998) and, in particular, anarticle commencing at page 429 of this book by Takumi Ueno on “Chemistryof Photoresist Materials.” As is disclosed on pages 433-434 of theSheats et al. book, the properties of positive photoresist vary with “.. . the characteristics of the novolak resins, such as the isomericstructure of cresol, the position of the methylene bond, the molecularweight, and the molecular weight distribution” (at page 433).

[0054] Novolak resins are thermoplastic phenol/formaldehyde condensationproducts formed by the condensation of cresol with formaldehyde.Depending upon the cresol used, one may obtain the methylene bond in themeta position (by using 3-methylphenol), and/or the ortho position (byusing 2-methylphenol), and/or the para position (by using4-methylphenol).

[0055] Shipley does not disclose for its “MICROPOSIT S1800 SERIES PHOTORESISTS” any information relating to “. . . the characteristics of thenovolak resins, such as the isomeric structure of cresol, the positionof the methylene bond, the molecular weight, and the molecular weightdistribution.”

[0056] Referring again to FIG. 2, and in the preferred embodimentdepicted therein, it will be seen that grating forming layer 14 iscomprised of a base layer 22 which, preferably, is at least about 0.25microns thick, as well as the actual surface-relief grating 15.

[0057] The surface-relief grating 15 depicted in FIG. 2 is preferablyperiodic, that is substantially the same shape is repeated. In thepreferred embodiment depicted in FIG. 2, the grating has a groovefrequency (“G”) of from about 400 to about 1,100 grating lines perlinear millimeter. In one embodiment, there are from about 500 to about900 grating lines per linear millimeter.

[0058] The distance between adjacent grating line 24 peaks (or valleys)is referred to as the grating line spacing D and is shown in FIG. 2. Dis the reciprocal of G, the groove frequency and, thus, ranges fromabout 0.91 to about 2.5 microns and, preferably, from about 1.11 toabout 2.0 microns.

[0059] The peak height of the lines 24, “h,” as shown in FIG. 2, is themaximum distance from the trough to the peak of the grating lines 24. Ingeneral, h ranges in height from about 0.5 microns to about 5 microns.

[0060] The ratio of h to D, which is also referred to as the gratingaspect ratio, and in the preferred transmission grating embodiment has avalue of about 1.3 to about 2.0. In another embodiment in which asurface-relief reflection grating is used, the h/D ratio is preferablyfrom about 0.3 to about 0.4 for the reflection grating element.

[0061] Referring again to FIG. 1, in this preferred embodiment thegrating 15 is a plane diffraction grating having parallel, equidistantlyspaced grating lines which reside on a flat surface. When one looks downonto the grating surface of the grating element 10, he will see amultiplicity of parallel grating groove lines spaced equidistantly fromeach other. As is known to those skilled in the art, one of theproperties of a plane diffraction grating, as described above, is thatit does not introduce optical power into the diffracted beam, i.e., acollimated incident beam is diffracted as a collimated beam.

[0062] The grating 15 is believed to be substantially more durable, whentested by a specified test, than are comparable prior art photoresistsurface-relief diffraction gratings. The test used to evaluate thedurability of grating 15 is set forth in Bellcore publicationGR-1221-CORE, issue 2, January, 1999, entitled “Generic ReliabilityAssurance Requirements for Passive Optical Components.” At page 6-4 ofthis publication, a “High Temperature Storage Test (Damp Heat)” isdescribed. This test requires that the item tested, when subjected to atemperature of 85 degrees centigrade and a relative humidity of 85percent, have less than 0.5 decibel optical insertion loss variationafter being tested for at least 500 hours. The functionality of the itemunder test is periodically evaluated. A change of 0.1 decibel in opticalinsertion loss corresponds to a change of 2.27 percent in the opticalperformance of the item while a 0.5 decibel change corresponds to a10.875 percent change in the optical performance of the item.

[0063] The preferred diffraction grating element 10 of this inventionmeets the aforementioned Bellcore test requirements for at least about1,700 hours. By comparison, a prior art photoresist surface-relieftransmission diffraction grating element identical in every manner butthe means in which the preferred photoresist grating element 10 ispost-processed, fails the aforementioned Bellcore test completely inless than about 2 hours, that is, the grating surface structurecompletely disappears in less than 2 hours at these test conditions.

[0064] The production of a diffraction grating assembly from photoresistis well known to those in the art. See, e.g., U.S. Pat. No. 4,289,371.

[0065] One preferred process for preparing the grating element 10 ofthis invention is disclosed in FIG. 3. Referring to FIG. 3, and in thepreferred process described therein, in step 50 photoresist is appliedto the top surface of a substrate. The substrate preferably is eitherrectangular or circular in shape and, most preferably, is made from theULE glass material described elsewhere in this specification. In onepreferred embodiment, the substrate is about 3.5 millimeters thick.

[0066] The preferred photoresist material, which is also describedelsewhere in this specification, is spread over the top surface of thesubstrate to a uniform thickness, preferably by a spin coating method inwhich the substrate is rotated at a speed of from about 2,000 to about4,000 revolutions per minute and the photoresist is spread and dried bycentrifugal force. In one embodiment, the photoresist is applied to athickness of about 3 microns.

[0067] Thereafter, in step 52 of the process, the coated substrate isheated to vaporize any remaining solvent in the coating. It is preferredto place the substrate onto a hot plate preheated to a temperature ofabout 110 degrees centigrade and to so heat the coated substrate for aperiod of from about 2 to about 10 minutes.

[0068] Thereafter, in step 54, the substrate is removed from the hotplate and allowed to cool under ambient conditions for at least about 5hours.

[0069] Thereafter, in step 56, the substrate is exposed to aholographically generated optical interference pattern. Such opticalinterference pattern is preferably produced by two interferingcollimated laser beams which are derived from the same helium cadmiumlaser operating at a wavelength of 442 nanometers. The angle subtendedby the interfering beams determines the period of the interferencepattern and, thus, the period of the final diffraction grating.Reference may be had to an article by Fujio Iwata et al. entitled“Characteristics of Photoresist Hologram and its Replica,” AppliedOptics, Volume 13, number 6, pages 1327 et seq. (June, 1974). Referencealso may be had to a paper by H. Werlech et al. on “Fabrication of highefficiency surface-relief holograms” which was published in the Journalof Imaging Technology, 10(3):105 (1984). Reference may also be had tomany different United States patents which disclose surface-reliefholograms, including, e.g., U.S. Pat. Nos. 6,160,668, 6,157,474,6,067,214, 6,049,434, 6,017,657, 5,986,838, 5,948,199, 5,917,562,5,896,483, 5,889,612, 5,961,990, 5,856,048, 5,838,466, 5,790,242,5,786,910, 5,757,523, 5,757,521, 5,756,981, 5,748,828, 5,742,411,5,712,731, 5,691,831, 5,691,830, and the like; the entire disclosure ofeach of these United States patents is hereby incorporated by referenceinto this specification.

[0070] Thereafter, in step 58, the exposed latent image in thephotoresist layer is developed by submerging the exposed photoresist indeveloper. A similar process for preparing, exposing and developingphotoresist coated substrates is disclosed on the world wide web athttp://www.ece.gatech. edu/research/labs/vc/processes/photoLith.html.

[0071] One may use conventional developing solutions such as, e.g., oneor more of the photoresist developers disclosed in U.S. Pat. Nos.6,087,655, 6,067,154, 5,881,083, 5,805,755, 5,607,800, 5,521,030,5,113,286, 4,826,291, 4,804,241, 4,725,137, 4,617,252, 4,589,972,4,566,889, 4,505,223, 4,469,544, 4,236,098, 4,204,866, 4,157,220,3,945,825, 3,944,420, and the like. The entire disclosure of each ofthese United States patents is hereby incorporated by reference intothis specification.

[0072] In one embodiment, the photoresist developer used is Shipley's“Microposit 303A Developer.” This Shipley developer is believed to be asodium hydroxide solution at a pH of about 14.

[0073] After the photoresist has been exposed to the developer, it isrinsed with filtered deionized water and spun dry in step 60 at a speedof about 500 revolutions per minute for about 1 minute until the gratingsurface appears dry.

[0074] The grating is then inspected, using a laser beam, to determineits diffraction efficiency. This diffraction efficiency information maybe used to adjust either the exposure time and/or development time sothat the process conditions for subsequently produced gratings may beadjusted and controlled.

[0075] The steps 50 through 60 describe one set of conditions for makinga photoresist surface-relief diffraction grating element. A descriptionof some of the technology involved in these steps 50 to 60 is set forthat pages 515 to 565 of the aforementioned James R. Sheats et al. book inan article by Bruce R. Smith entitled “Resist Processing.”

[0076] The steps 62 et seq. describe critical post-exposure/developmentsteps for insuring that the ultimate grating produced has improveddurability properties, as measured by the aforementioned Bellcore test.Prior to discussing the steps 62 et seq., which produce the desireddurable grating, applicant will discuss the post-processing treatmentsuggested by the prior art.

[0077] At page 562 to 563 of the aforementioned James R. Sheats et al.book, it is disclosed that: “Novolac resins generally suffer fromthermal distortion . . . To enhance the thermal properties ofDNQ/novolac resins, the UV crosslinking poperties of novolac can beutilized. Although the efficiency is quite low, novolac resin can bemade to crosslink at DUV wavelengths. This is facilitated at hightemperatures . . . By elevating the temperature of the “DUV” cure”process, oxidation of the bulk of the resist feature can beaccomplished.”

[0078] In accordance with the suggestion made in the James R. Sheats etal. book, applicant conducted an experiment in which the grating waspost-processed by subjecting the grating to a high temperature (inexcess of 100 degrees centigrade) while simultaneously subjecting thegrating to ultraviolet light in the spectrum range of from about 200 toabout 320 nanometers. As a result of these experimental conditions, thetreated photoresist material became unacceptably dark.

[0079] In the preferred process of this invention, when the photoresistis sequentially subjected to the ultraviolet light exposure andthereafter subjected to high temperature, not only is a durable gratingproduced, not only does the treated photoresist material not becomedark, but the treated photoresist material becomes clearer and opticallymore desirable.

[0080] In step 62 of the process, the photoresist surface of thedeveloped diffraction grating is at ambient room temperature andpressure conditions directly exposed to a lamp producing ultravioletlight in the spectrum range of from about 200 to about 320 nanometers,which is often referred to as deep ultraviolet (DUV) radiation. Forexample, one may use one or more of the lamps disclosed in U.S. Pat.Nos. 4,389,482, 4,344,008, 4,312,934, 4,299,911, and 4,049,457, theentire disclosure of each of which is hereby incorporated by referenceinto this specification.

[0081] Unlike prior processes, there is no intermediate material, exceptair, positioned between the DUV light source and the photoresist. Thedistance between the DUV lamp and the unprotected photoresist surface isgenerally from about 6 to about 10 inches. One may use, e.g.,conventional germicidal lamps for this purpose such as, e.g., GermicidalLamp FG15T8. The photoresist surface is exposed to such ultravioletlight for about 10 minutes.

[0082] The Germicidal Lamp FG15T8 is 18 inches long, operates at 0.3amperes and 56 volts, has a nominal lamp wattage of 15 watts, provides3.5 watts of ultraviolet radiation at 253.7 nanometers, and has anaverage life of 8,000 hours.

[0083] Exposure of the photoresist grating forming layer to the DUVlight source at ambient room temperature and pressure conditionsbleaches the photoresist layer and, thereby changes the color of thephotoresist layer from a yellow color to a substantially optically clearcolor having no visible observable color tint. This substantiallyoptically clear color is sometimes referred to as water white. Thephotoresist layer stays substantially optically clear not only afterbeing heated in step 64 of the process but also after being tested forabout 1,000 hours at the previously described Bellcore test conditions,whether the grating surface is uncovered or encapsulated as illustratedin FIG. 9 of this specification. It is noticed that an uncovered gratingdevelops a very light yellow tint color after about 1,700 hours oftesting at the Bellcore test conditions. This yellow tint color does notappear to affect the diffraction efficiency performance of the gratingwhen tested at laser wavelength of 633 nanometers.

[0084] Shipley, in their technical marketing data sheets for the ShipleyS1813 Photo Resist product, presents data that shows that exposure ofthis photoresist to light sources having a wavelength spectrum fromabout 350 to 450 nanometers changes the optical absorption of theresist, particularly for optical wavelengths of less than about 500nanometers. Applicant has observed that photoresist surface-relieftransmission gratings exposed to light only in the 350 to 450 nanometerspectrum range become substantially optically clear but fail in lessthan about 2 hours when tested at the aforementioned Bellcore testconditions. It also has been observed that gratings exposed only tolight in the 350 to 450 nanometer spectrum range fail within about 30minutes when placed in a dry heat (<10 percent relative humidity) ovenhaving a temperature greater than about 110 degrees centigrade. It alsohas been observed that gratings exposed only to light in the 350 to 450nanometer spectrum range develop a yellow color over time if the gratingsurface is not encapsulated as illustrated in FIG. 9 of thisspecification, even when the uncovered grating elements are left atambient room temperature and humidity conditions. It also has beenobserved that gratings exposed to the DUV light source but notundergoing the bake step 64 of the preferred process, appear to pass theaforementioned Bellcore test but turn a yellow color only after about160 hours of test time at the aforementioned Bellcore test conditions.

[0085] In step 64 of the process, the DUV exposed photoresist surface isthen heat treated by being heated in a relatively dry oven to atemperature of from about 110 to about 150 degrees centigrade,preferably for at least about 30 minutes. It is preferred that thephotoresist surface be placed into a preheated oven at the desiredtemperature of from about 110 to about 150 degrees centigrade.

[0086] For surface-relief transmission grating elements that undergosteps 62 and 64 of the process presented in FIG. 3, the grating elementsafter being tested at the aforementioned Bellcore test conditions of 85degrees centigrade and 85 percent relative humidity for at least 1,000hours, have optical performance as measured by their diffractionefficiency values for light of 633 nanometers and S optical polarizationthat is within 6 percent of the optical performance they had prior tobeing tested at the Bellcore test conditions. Therefore, gratingelements undergoing steps 62 and 64 of the process pass theaforementioned Bellcore test conditions since this Bellcore test deemsthat an item passes these test conditions if its optical performancedoes not change by more than 0.5 decibels (10.87 percent) after beingtested for 500 hours at these test conditions.

[0087] It should be noted that the grating fabrication processing steps50 through 64 of FIG. 3 can be utilized to produce both surface-relieftransmission gratings and surface-relief reflection gratings. Surfacerelief transmission gratings fabricated with these processing steps areexposed and developed so that the finished grating has a grating aspectratio in the range of about 1.3 to about 2.0 while surface-reliefreflection gratings fabricated with these processing steps are exposedand developed so that the finished grating has a grating aspect ratio inthe range of about 0.3 to about 0.4. After step 64 of the process, thegrating surface of a reflection grating is coated with a reflectingmetal film, such as gold or aluminum. The reflective metal film isusually applied to the grating surface using either evaporation orelectronic beam sputtering techniques that are performed in a vacuumchamber.

[0088] The finished diffraction grating has certain unique properties.It has the durability property and substantially optically clear colorproperty that are described elsewhere in this specification. It alsopreferably has a diffraction efficiency of greater than 70 percent foroptical wavelengths in the 1280 to 1620 nanometer spectrum region thatfiber-optic communication systems use. It also preferably hasessentially equal diffraction efficiency values for S and P polarizedoptical components, that is, the S and P polarizations have diffractionefficiency values within about 5 percent of each other.

[0089]FIG. 4 presents measured diffraction efficiency data for asinusoidal surface-relief transmission grating formed in photoresist.This data is presented as a function of λ/D which is the ratio of theoptical wavelength, λ, of the beam incident on the grating to thegrating line spacing D, and for the Littrow diffraction condition, thatis θ_(i)=θ_(d). The definition used to calculate the grating diffractionefficiency data values in FIG. 4, and used in this specification inreference to diffraction efficiency, is that the diffraction efficiencyof a grating element is the ratio of the intensity of the first orderdiffracted beam divided by the intensity of the beam incident to thegrating diffracting surface for either the S or P polarized opticalcomponent of the beam. This definition of diffraction efficiency doesnot account for optical insertion losses in the grating element due toreflection losses at the substrate non-grating surface or by opticalabsorption with the substrate material. These substrate related opticalinsertion losses can be minimized by using antireflection coatings onthe non-grating substrate surfaces and/or by using substrate materialshaving low optical absorption for the wavelength spectrum used with thegrating element. The measured data points are shown on the graph in FIG.4 as open circles for both polarization components, while the drawncurves represent the best fix to this data. In this specification, whendiscussing the performance of a transmission diffraction grating, wewill use the American polarization convention, that is, S polarizedlight has its electric field parallel to the grating lines while Ppolarized light has its electric field perpendicular to the gratinglines. To achieve the high diffraction efficiency values presented inFIG. 4, the surface-relief transmission grating must have a deep grooveprofile shape, that is, the grating aspect ratio must be between about1.3 and 2.0.

[0090] A more detailed discussion of the diffraction efficiencyproperties of a surface-relief transmission grating is presented atpages 179-182 of the Loewen text discussed elsewhere in thisspecification. As shown on these pages of this text, when the incidentand diffracted optical beams to a surface-relief transmission gratinghave substantially equal angles with respect to the normal to thegrating surface the S polarized optical component has a diffractionefficiency of at least 70 percent for λ/D ratios of from about 0.8 toabout 2.0 while the P polarized optical component has a diffractionefficiency of at least 70 percent for λ/D ratios of from about 0.8 toabout 1.2 and has a value that decreases from at least 70 percent whenthe λ/D ratio is about 1.2 to a value of less than 10 percent when theλ/D ratio is about 1.43 to about 2.0.

[0091] Examination of FIG. 4 reveals that essentially equal diffractionefficiency values for both S and P polarization optical components canbe achieved with a transmission sinusoidal surface-relief grating havinga λ/D range of about 0.8 to approximately 1.2. Also, as the FIG. 4 datashows, surface-relief transmission gratings can achieve diffractionefficiency values of greater than 90 percent for both S and Ppolarizations for the 0.8 to 1.2 λ/D ratio range and do not exhibit theanomalies in diffraction efficiency performance as a function of λ/Dratio that are observed with reflecting surface-relief gratings. Thehigh diffraction efficiency and lack of anomalies observed withsurface-relief transmission gratings occur because they do not containmetal and, therefore, do not have in the visible or near infraredspectrum region complex absorption properties that are characteristic ofmetal coated surface-relief reflection gratings.

[0092] In one embodiment, it is preferred that grating 15 be operated sothat the incident and diffracted beams have substantially equal angleswith respect to the normal to the grating surface, that is, being withinabout 15 degrees of the Littrow diffraction condition.

[0093] In one embodiment, the diffraction grating 15 is operated so thatthe diffraction efficiency values for both the S and P polarized opticalcomponents are within 10 percent of each other, which corresponds tooperating a surface-relief transmission grating with a λ/D ratio of fromabout 0.8 to about 1.2.

[0094] Using a surface-relief transmission grating element at a λ/Dratio value higher than about 1.2 results in the grating havingdifferent diffraction efficiency values for the S and P polarizationcomponents (see FIG. 4). Using diffraction grating-based devices thathave different diffraction efficiency values for the S and Ppolarizations may significantly increase the polarization dependentnoise level of fiber-optic communications systems incorporating thedevices. In a grating-based device using a surface-relief transmissiongrating having a λ/D value greater than about 1.2, this polarizationdependent noise problem can be ameliorated by incorporating polarizationcontrolling optical elements into the device so that the device achievesessentially equal radiometric throughput efficiency values for the S andP polarized optical components. Radiometric throughput efficiency for acomponent, device or a system is defined as the ratio of the intensityof the optical beam exiting the component, device or system divided bythe intensity of the optical beam incident to the component, device, orsystem and is usually measured for each optical polarization component.

[0095] The polarization dependent noise level of a fiber-opticcommunication system is increased whenever a device having greater thanabout 5 percent difference between its radiometric throughput efficiencyvalues for S and P polarized optical components is incorporated into thesystem. This increase in polarization dependent noise level occursbecause the optical beams propagating in fiber-optic communicationsystems have no defined polarization direction and continually changepolarization direction as a function of time.

[0096] Because the polarization dependent noise level of a device usedin a fiber-optic communication system is determined by the differencebetween its radiometric throughput efficiency values for S and Ppolarized optical components, the preferred grating-based deviceembodiments in this specification are operated to have radiometricthroughput efficiency values for S and P polarizations that are equal towithin about 5 percent of each other. This is accomplished in some ofthe preferred grating-based device embodiments by using surface-reliefdiffraction grating elements that have essentially equal diffractionefficiency values for S and P optical polarizations. While notspecifically stated for each of the preferred grating-based deviceembodiments in this specification, the other optical components used inthese devices, such as fibers, lenses, mirror reflecting surfaces,non-grating transmitting surfaces, etc., incorporate thin film opticalcoatings that not only improve the radiometric efficiency performance ofthe component, and therefore the device, but also ensure that thesecomponents have radiometric throughput efficiency values for S and Poptical polarization components that are equal to within about 5 percentof each other. If the grating element used in the device has adiffraction efficiency difference of up to about 10 percent between theS and P polarized optical components, its radiometric throughputinefficiency difference for S and P polarizations can be compensated forby incorporating optical components into the device that have theopposite radiometric throughput inefficiency difference with regard tothe S and P polarization components. When the grating element used inthe device has a difference of greater than about 15 percent between theS and P polarized optical components, the device incorporatespolarization controlling optical elements that enable the device toachieve radiometric throughput efficiency values for S and Ppolarizations that are equal to within about 5 percent of each other.

[0097] To summarize the preceding statements, the preferredgrating-based device embodiments in this specification are configured sothat the optical components of the device function as an opticallyintegrated assembly so that the device achieves radiometric throughputefficiency values for S and P polarized optical components that areequal to within about 5 percent of each other.

[0098]FIG. 5 schematically illustrates a preferred embodiment in whichthe surface-relief transmission grating element 10 is incorporated intoa spectrophotometer device 80 used as part of an on-line wavelengthchannel monitoring system capable of obtaining information about theoptical power, wavelength and optical-signal-to-noise ratio of eachwavelength signal channel in a wavelength-division multiplexing (“WDM”)fiber-optic communication system.

[0099] As depicted in FIG. 5, input optical wavelength channel signalinformation is delivered to device 80 by transmission fiber 82. Inputtransmission fibers are well known to those skilled in the art and aredisclosed, e.g., in U.S. Pat. Nos. 6,151,145, 5,798,855, 5,790,285,5,745,613, 5,532,864, 5,452,124, 5,377,035, and the like. The entiredisclosure of each of each of these United States patents is herebyincorporated by reference into this specification.

[0100] The input transmission fiber 82 to the spectrophotometer monitordevice 80 contains λ₁, λ₂, and λ₃ wavelength channel signals which exitfrom the end of the fiber as a diverging optical ray bundle 84. Thecollimating lens assembly 86 receives the ray bundle 84 diverging fromthe end of the input fiber 82 and converts it into a collimated beam 88which is incident on the transmission grating element 10. After beingdiffracted by element 10 the incident beam 88 is separated into λ₁, λ₂,and λ₃ wavelength channel beams 90 which propagate at slight angles withrespect to each other in the plane which is perpendicular to thediffraction grating lines of element 10, which FIG. 5 resides in. Thefocusing lens assembly 92 receives the angularly separated collimatedλ₁, λ₂, and λ₃ wavelength beams 90 from the grating element 10 andfocuses these beams onto the surface of the photodetector linear arrayelement 94.

[0101] The spatially separated focused wavelength channel beams 96 areincident on different photosensitive elements (not shown) in thephotodetector array 94 and, thereby, generate an independent electricalsignal 98 for each wavelength channel beam. The amplitude of eachelectrical signal 98 is proportional to the average light intensity ofthe wavelength channel beam incident on the photodetector elementgenerating that signal. While only three wavelength channel beams areshown being used with the monitoring device 80 of FIG. 5, it is evidentthat device 80 can be used with many more wavelength channel beams. AnInGaAs (indium gallium arsenide) photodetector array will normally beincorporated into wavelength monitoring devices used for communicationsystems operating in the 1280 to 1620 nanometers spectrum region.Commercial InGaAs photodetector arrays are available with 128, 256, and512 photodetector elements having either 25 or 50 micron spacing betweenelement centers. InGaAs photodetector fibers are well known to thoseskilled in the art and are disclosed, e.g., in U.S. Pat. Nos. 5,838,470,5,714,773, 5,428,635, 5,386,128, 5,055,894, 4,879,250, and the like. Theentire disclosure of each of these United States patents is herebyincorporated by reference into this ,specification.

[0102] In the device 80 of FIG. 5 the optical components are enclosedwithin a housing 100, which protects the optical components fromcontaminants. It is preferred that the housing 100 be comprised ofcomponents which do not adversely affect the performance of the opticalcomponents over the 70 degree operating temperature range specified forfiber-optic devices.

[0103] The spatial separation between the focused wavelength channelbeams 96 at the surface of the photodetector array 94 of device 80 isproportional to the diffracted angular separation between wavelengthchannel beams 90. The angle through which each wavelength beam isdiffracted by the grating element 10 can be calculated using the gratingequation: $\begin{matrix}{{{{\sin \quad \theta_{i}} + {\sin \quad \theta_{d}}} = \frac{m\quad \lambda}{D}},} & (1)\end{matrix}$

[0104] where θ_(i) and θ_(d) are, respectively, the angles that theincident and diffracted beams make with respect to the grating surfacenormal, λ is the optical wavelength of the incident and diffracted beam,D is the grating line groove spacing, and m is the diffraction order (orspectral order) which is an integer (m=0, ±1, ±2 . . . ). For thedevices shown in this specification it will be assumed that we are usingthe first diffracted order beams, that is, m=1. The diffracted angularseparation, dθ_(d), between the wavelength beams 90 is calculated bydifferentiating Equation (1) with respect to dλ, which gives:$\begin{matrix}{{d\quad \theta_{d}} = {\frac{d\quad \lambda}{D\quad \cos \quad \theta_{d}} = {\frac{\lambda}{D\quad \cos \quad \theta_{d}}{\frac{d\quad \lambda}{\lambda}.}}}} & (2)\end{matrix}$

[0105] I have separated out the λ/D cos θ_(d) and the dλ/λ ratio termsin Equation (2) to emphasize that the first ratio is a measure of thewavelength dispersion power of the diffraction grating element 10 whilethe second ratio indicates the wavelength resolving power required ofthe element 10. The spatial separation, W, between the focusedwavelength channel beams 96 at the surface of the photodetector array 94of device 80 is given by: $\begin{matrix}{{W = {{f\quad {\tan \left( {d\quad \theta_{d}} \right)}} = {\frac{f\quad d\quad \lambda}{D\quad \cos \quad \theta_{d}} = {\frac{f\quad \lambda}{D\quad \cos \quad \theta_{d}}\frac{d\quad \lambda}{\lambda}}}}},} & (3)\end{matrix}$

[0106] where f is the focal length of the focusing lens assembly 92 usedin device 80. For devices of interest, tan(dθ_(d)) is accuratelyapproximated by using the first term in its Taylor series, as indicatedin Equation (3).

[0107] Examination of Equation (3) reveals that as the spacing betweenwavelength channels, dλ, decreases in a WDM fiber-optic system, from 3.2to 1.6 to 0.8 to 0.4 nanometers, corresponding to a frequency decreasein channel spacing of from 400 to 200 to 100 to 50 GigaHertz, thespatial separation between the wavelength beams 96 at the surface of thephotodetector array 94 proportionally decrease. Because the spacingbetween the photosensitive elements of the photodetector array 94 areessentially fixed at 25 or 50 micrometers, the focal length of thecollimating/focusing lens assembly 92 must be increased, and/or thegrating element 10 must incorporate a finer grating line spacing (higherλ/D ratio) to accommodate the decreases that are occurring in thewavelength spacing of WDM fiber-optic systems. Increasing the focallength of the focusing lens assembly 92 of the device 80 has a number ofundesirable associated results. These undesirable results include: thescaling of lens aberrations with focal length, which increases thefocused spot sizes of the wavelength beams 96 at the photodetector array94; device performance stability becomes more sensitive with regard toboth mechanical and thermal induced changes; and the size of the deviceincreases, which is opposite to the trend for fiber-optic communicationdevices. Increasing the λ/D ratio of the grating element 10 used in thedevice usually has the undesirable result of having the grating element10 have significantly different diffraction efficiency values for the Sand P polarized components of the optical beam, which significantlyincreases the polarization dependent noise level of fiber-optic systemsincorporating such grating elements.

[0108] Therefore, it is desired to increase the dispersion of thegrating element 10 used in the device 80 while still achievingessentially equal diffraction efficiency values for the S and Ppolarization components of the optical beam. To this end, one can use aλ/D ratio of about 1.2 and still achieve essentially equal diffractionefficiency values for the S and P polarized optical components and,according to Equation (3), one can increase the dispersion power ofgrating element 10 by increasing the diffraction angle θ_(d) used in thedevice.

[0109] The required focal length of the focusing lens assembly 92 usedin the FIG. 5 monitor device 80 can be calculated with Equation (3). Forexample, if it is assumed that the spatially separated focusedwavelength channel beams 96 in device 80 are incident on adjacentphotosensitive elements in the photodetector array 94 and that theseelements have a 50 micron spacing between element centers, that thedevice 80 incorporates a surface-relief transmission grating element 10having a λ/D ratio of 1.0 for an optical wavelength of 1550 nanometers,and that θ_(i)=θ_(d)=30 degrees for a wavelength of 1550 nanometers,then the focusing lens assembly 92 used in device 80 must have a focallength of 83.9 millimeters when used with a WDM fiber-optic systemhaving 0.8 nanometers (100 GigaHertz) spacing between wavelengthchannels. One could reduce the focal length for this WDM systemrequirement down to approximately 60 millimeters by using asurface-relief transmission grating element 10 having a λ/D ratio of1.15 that operated with θ_(i)=26.3° and θ_(d)=45°. The focal length ofthe focusing lens assemblies used in spectrophotometer units used tomonitor the performance of WDM fiber-optic systems having 40 or morechannels cannot be made much shorter than 60 millimeters, since the lensassemblies must provide good imaging performance across a photodetectorarray surface area that is 2 millimeters or more in length.

[0110] One may physically shorten the FIG. 5 monitoring device 80 bypositioning a beam fold mirror element after the transmission gratingelement 10 in device 80 such that the mirror reflects the diffractedbeams essentially parallel to the input beam path 88, as depicted inFIG. 6. The wavelength channel monitoring device 120 in FIG. 6 functionsexactly as described for the monitoring device 80 in FIG. 5. In additionto incorporating beam fold mirror element 122, the device 120 has beenmodified relative to the device 80 in FIG. 5 in several ways. Only asingle wavelength beam 90 is depicted in device 120. The focal length ofthe focusing lens assembly 92 in device 120 is significantly longer thanthe focal length for the collimating lens assembly 86 used in thisdevice. The collimating and focusing lens assemblies in device 120 aredepicted as air spaced doublets vs. the air spaced triplet lensassemblies depicted in device 80 of FIG. 5. The transmission gratingelement 10 in device 120 is depicted as functioning with θ_(i)=28degrees and θ_(d)=45 degrees, while the transmission grating element 10in device 80 of FIG. 5 is depicted as functioning with θ_(i)=θ_(d)=30degrees. Device 120 incorporates an internal light baffle element 124 toshield the photodetector linear array element 94 from any back-scatteredlight originating in the input beam path prior to and including thegrating element 10. The size of the photodetector linear array element94 in FIG. 6 is depicted considerably larger than the correspondingelement in FIG. 5 to more accurately reflect the dimensions of currentcommercially available InGaAs linear array elements.

[0111] Because the collimating and focusing lens functions in the FIGS.5 and 6 wavelength monitoring devices are separate, one can optimize thelenses used for these imaging functions and thereby potentially improveupon the cost/performance ratio of the device. While the collimating andfocusing lens assemblies in FIGS. 5 and 6 are depicted as composed ofeither three or two conventional singlet lens elements, one could usefewer conventional spherical lens elements, and/or lens elements havingaspheric surfaces and/or gradient index based lens elements, such as aSELFOC lens (sold by NSG America, Inc. of Somerset, N.J.) for these lensassemblies. One could also use a combination of lens and mirrorelements, or just mirror elements, to construct the collimating andfocusing lens assemblies depicted in FIGS. 5 and 6. The collimating lensassemblies used in the FIGS. 5 and 6 devices can have a simpler lensassembly configuration than used for the focusing lens assemblies inthese devices since the collimating lens function only on axis.

[0112] Appropriate lens assembly combinations will be apparent to thoseskilled in the art, as described in the following patents. Typicalcollimating lens assemblies are disclosed in U.S. Pat. Nos. 6,279,464,6,169,630, 6,137,933, 6,028,706, 6,011,885, 6,011,884, 6,008,920,4,852,079, 4,405,199, and the like. Focusing lens assemblies aredisclosed, e.g., in U.S. Pat. Nos. 6,167,174, 6,097,860, 6,097,025,6,094,261, 6,075,592, 5,999,672, 5,793,912, 5,450,510, 5,450,223,5,440,669, 5,026,131, 4,479,697, and the like. The entire disclosure ofeach of these United States patents is hereby is incorporated byreference into this specification.

[0113] The focusing lens assembly 92 in device 120 of FIG. 6 is depictedas having a focal length that is in the range of 2 to 3 times longerthan the focal length used for the collimating lens assembly 86 used inthis device. This 2 to 3 difference in focal lengths between these lensassemblies can be used because the InGaAs photodetector linear arrays 94used in the monitoring devices 80/120 have an element cell size in therange of 25 to 50 microns, while the input fibers 82 used in thesedevices have a core diameter in the range of 8 to 9 microns. Because ofthe 3 to 1 or greater ratio between input fiber core diameter andphotosensitive element cell size for the FIGS. 5 and 6 monitoringdevices, one can use a collimating lens assembly in these devices havinga focal length which is only approximately one-third of the focal lengthused for the focusing lens assembly incorporated in these devices andthereby optimize the numerical aperture (NA) imaging performance foreach lens assembly, which should improve the cost/performance ratio ofthe device.

[0114] While the inclusion of the beam fold mirror element 122 in device120 of FIG. 6 reduces the physical size of the wavelength monitoringdevice relative to the embodiment illustrated in FIG. 5, it does notchange the wavelength dispersion properties of the device relative tothat achieved with the FIG. 5 device. The beam fold mirror element 122can be configured so that it not only reduces the size of the device butalso effectively doubles the wavelength dispersion power of thetransmission grating element 10 used in the device.

[0115]FIG. 7 schematically illustrates how a beam fold mirror element122 can be utilized so that the incident beam 84 makes a dual passthrough the surface-relief transmission grating element 10 and therebydoubles the wavelength dispersion power of the grating element 10. Theincident beam 84 to the transmission grating element 10 in device 130 ofFIG. 7 contains λ₁ and λ₂ wavelength components. After the incident beamis diffracted by the grating element 10, these optical wavelengthcomponents are angularly separated by Δθ. The mirror element 122 indevice 130 is angularly orientated so that the λ₁ wavelength beam 132 isretroreflected back on itself Because the grating element 10 functionsin a reversible manner, the grating element 10 rediffracts theretroreflected λ₁ wavelength 132 beam back along the direction of theincident beam 84 as beam 134. The dual pass transmission gratingarrangement of device 130 mimics a reflection grating element operatingat the Littrow condition for the λ₁ wavelength beam, since this beam isretrodiffracted back on itself After reflecting from the mirror element122, the λ₂ wavelength beam 136 in device 130 propagates back to thegrating element 10 where, due to its angle of incidence, the λ₂wavelength beam 136 is rediffracted from the grating element 10 with anangle equal to approximately 2Δθ relative to the propagating directionof the retrodiffracted λ₁ wavelength beam 134.

[0116] The angular separation, dθ_(dS), between beams 134 and 136 forthe double pass grating arrangement in FIG. 7 is calculated bydifferentiating Equation (1) with respect to dθ_(i), which gives:

dθ _(dS)=−[cos θ_(i)/cos θ_(d)] dθ_(i)=2 [cos θ_(i)/cos θ_(d)]dθ_(d),  (4)

[0117] where for the grating/mirror arrangement in FIG. 7, dθ_(i)=−2dθ_(d), since the angle between the λ₂ beam 136 incident on the mirrorelement 122 and the λ₂ beam 136 reflected from mirror element 122 is2Δθ. Equation (2) is used to calculate the value for dθ_(d). For thedual pass grating arrangement depicted in FIG. 7, θ_(i)≈θ_(d) and,therefore, dθ_(dS)≈2dθ_(d)=2Δθ.

[0118] In effect, the grating element 10 and mirror element 122combination in device 130 of FIG. 7 doubles the λ/D value of the gratingelement 10. By using the grating/mirror combination in device 130,surface-relief transmission grating-based devices can be constructedwith effective λ/D grating values of 1.6 to 2.4 while achievingessentially equal diffraction efficiency values for S and P polarizedoptical components. The high wavelength dispersion power provided bythese dual pass transmission grating-based devices provides significantadvantages when these devices are used in WDM fiber-optic communicationsystems.

[0119] The grating/mirror combination in device 130 of FIG. 7 achievesthe effective doubling of the λ/D value of grating element 10 bycascading the grating dispersion power of grating element 10, similar tothe narrowing of the spectrum band-pass width of an interferencewavelength selection filter device by the cascading of filter elements.This cascading of the grating dispersion power does not effect thewavelength filter function of the grating-based devices incorporatingthis cascaded grating arrangement, since the wavelength filter functionof these grating-based devices is determined by the physical dimensionsof the output array structures used in those devices. The onlysignificant negative associated with using this cascaded gratingarrangement is a decrease in device throughput radiometric efficiencyassociated with the optical power loss due to the beam being diffractedtwice by the grating element. It is estimated that greater than 80percent radiometric throughput efficiency can be achieved for both S andP polarized beam components propagating twice through a surface-relieftransmission grating element having a λ/D ratio value in the range of0.8 to 1.2 for optical wavelengths in the 1280 to 1620 nanometersspectrum range.

[0120] Referring again to FIG. 7, one can change the wavelength of thebeam 134 retrodiffracted back on itself, and thus change the wavelengthtuning parameters of the device 130, by rotating the mirror element 122in the direction of arrow 138 and/or arrow 140 by conventional means.This wavelength tuning property is well known and is used inconventional double-pass mirror-reflection grating-basedspectrophotometers, as discussed in an article by Ghislain Levesque inthe June, 2000 issue of Photonics Spectra (see FIG. 5 on page 110).

[0121] The dual pass transmission grating arrangement in FIG. 7 isaccomplished by using separately a grating element 10 and mirror element122. By comparison, and as illustrated in FIG. 8, a dual passtransmission grating device 150 can be fabricated using a singletransmission glass block element 152 that incorporates a surface-relieftransmission grating 15 and a reflecting mirror surface 154. The device150 functions as described for the device 130 in FIG. 7. As depicted inFIG. 8, a single wavelength beam 84 is incident on the dual pass gratingdevice 150 at the Littrow diffraction condition for the dual passarrangement depicted in device 150 and is retrodiffracted back along theincident beam path 84 as beam 134. As depicted in FIG. 8, thenon-optical transmitting and reflecting surfaces of the glass block 152have been coated with an optical absorption coating 156 that is designedto absorb the nondiffracted zeroth order beam energy and other scatteredlight which may occur within the glass block element 152. Opticalabsorption coatings are well known to those skilled in the art and aredisclosed, e.g., in U.S. Pat. Nos. 6,075,635, 5,893,364, 5,633,494, andthe like. The entire disclosure of each of these United States patentsis hereby incorporated by reference into this specification.

[0122]FIG. 9 schematically illustrates how the dual pass transmissiongrating device 150 of FIG. 8 can be fabricated using a surface-relieftransmission grating element 10 that is attached to the input opticaltransmitting surface of glass block element 152 incorporating reflectingmirror surface 154. The device 160 in FIG. 9 functions as described forthe device 130 in FIG. 7. As depicted in FIG. 9 a single collimatedwavelength beam 162 is incident on the dual pass grating device 160 atthe Littrow diffraction condition for the dual pass arrangement depictedin device 160 and is retrodiffracted back along the incident beam path162 as beam 166.

[0123] As depicted in FIG. 9, the device 160 is fabricated so that thegrating surface 15 of the grating element 10 is encapsulated between thesubstrate 12 of grating element 10 and the input optical transmittingsurface to the glass block element 152. A sealing element 168, such asepoxy, is used in device 160 to encapsulate the air gap layer 170 thatexists between the surface-relief transmission grating surface 15 andthe input optical transmitting surface of the glass block element 152.The main function of the sealing element 168 is to prevent contaminants,liquids or solvent vapors that could damage the grating surface fromentering the air gap layer 170. The encapsulated grating surfaceconfiguration of device 160 also protects the grating surface from beingdamaged due to handling and cleaning of the grating element. The inputoptical transmitting surfaces of both the grating substrate 12 and theglass block 152 are antireflection coated to minimize optical reflectionlosses at these surfaces. As depicted in FIG. 9, the non-opticaltransmitting and reflecting surfaces of the glass block 152 have beencoated with an optical absorption coating 156 that is designed to absorbthe nondiffracted zeroth order beam energy and other scattered lightwhich may occur within the glass block element 152. It is evident thatthe glass block elements depicted in FIGS. 8 and 9 can be made longer orshorter than what is depicted in these figures.

[0124] As is illustrated by the Examples set forth in thisspecification, devices made in accordance with FIG. 9 will pass theBellcore high humidity/high temperature tests only if certain specifiedadhesives are used.

[0125] Schematic top, side and isometric views in FIGS. 10A, 10B and10C, respectively, illustrate how the dual pass grating element 150 ofFIG. 8 can be incorporated into demultiplexer (Demux) device 180 used infiber-optic WDM systems. The input transmission fiber 182 to the Demuxdevice 180 contains λ₁, λ₂, and λ₃ wavelength channel signals which whenexiting from the end of the fiber 182 form a diverging optical raybundle 186 having a cone angle determined by the numerical aperture (NA)of the input fiber 182. The end of the fiber 182 is supported andpositioned by holder 183 at the focal plane of the collimating/focusinglens assembly 87. The collimating/focusing lens assembly 87 receives theray bundle 186 diverging from the end of the input fiber 182 andconverts it into a collimated beam 188 which is incident on the dualpass grating element 150. As depicted in FIG. 10A, the incident beamafter being diffracted by the dual pass grating element 150 is separatedinto λ₁, λ₂, and λ₃ beams 190 which propagate at slight angles withrespect to each other in the plane perpendicular to the diffractiongrating lines, which FIG. 10A resides in. In the plane parallel to thegrating lines, which FIG. 10B resides in, the dual pass grating element150 functions like a mirror in that the incident beam 188 and thedifferent diffracted wavelength beams 190 have essentially the sameangle with respect to the normal to the grating surface 15 in thisplane.

[0126] As depicted in FIGS. 10A and 10B, the collimating/focusing lensassembly 87 receives the collimated diffracted λ₁, λ₂, and λ₃ wavelengthbeams 190 from the duel pass grating element 150 and focuses these beamsonto the surface of the output fiber array 184. As indicated in FIG.10A, and more clearly in FIG. 10C, the output fiber array 184 consistsof individual fibers 192, 194, and 196 arranged in a row structure, theorientation of that row being parallel to the plane that isperpendicular to the grating lines. In the plane of the row the fibersare essentially evenly spaced by a distance W that is equal to theproduct of the focal length of the collimating/focusing lens assembly 87and the diffracted angular separation between wavelength beams 190, ascalculated using Equation (3). It should be noted that, when usingEquation (3) to calculate the W value for the dual pass diffractiongrating-based device illustrated in FIGS. 10A, 10B, and 10C, one mustaccount for the dual pass nature of the device, that is, one must usedθ_(dS) in place of dθ_(d) in Equation (3). The output fiber array 184is spatially positioned so that each of the spatially separated focusedwavelength channel beams 191 is incident on its corresponding outputfiber in the array 184. Essentially all of the light incident on thecore are of an output fiber in the array 184 is coupled into the fiberand transmitted to a separate photodetector device (not shown) thatprovides an electrical data signal corresponding to the informationtransmitted on that wavelength channel.

[0127] In the preferred embodiment depicted in FIGS. 10A, 10B and 10C,the optical components are enclosed within a housing 100, which protectsthe optical components from contaminants. It is preferred that thehousing 100 be comprised of components which do not adversely affect theperformance of the optical components over the 70 degree operatingtemperature range specified for fiber-optic devices.

[0128] The Demux device depicted in FIGS. 10A, 10B and 10C functions ina reversible manner, that is, the device can be used to opticallycombine different wavelength channels onto a single output fiber,thereby functioning as a multiplexer (Mux) device. While the precedingdiscussion of the Mux/Demux operating principles of the grating-baseddevice illustrated in FIGS. 10A, 10B, and 10C is limited to threewavelength channels, it is evident that this device can be used withmany more wavelength channels. The only component in the device thatneed be changed when the number of wavelength channels is changed is thenumber of fibers contained in the fiber-optic array holder 184. Thegrating-based device illustrated in FIGS. 10A, 10B and 10C can be usedto simultaneously Mux and Demux wavelength channels and thereby be usedto construct a bi-directional fiber-optic network system which providesdramatic cost savings in local and metro area networks not incorporatingin-line optical amplifiers. One method for achieving this bi-directionaloperation is by having adjacent wavelength channels be transmitted inopposite directions. This adjacent counter-propagating wavelengthchannel arrangement minimizes cross-take between both co-propagating andcounter-propagating wavelength channels and still enables theinput/output fiber-optic array holder to be constructed with essentiallyequal spacing between fibers.

[0129] Schematic top, side and isometric views are, respectively,presented in FIGS. 11A, 11B and 11C of an on-line wavelength channelsignal monitoring device 200 that utilizes dual pass grating element 150of FIG. 8. Comparison of the device of FIGS. 10A, 10B and 10C with thedevice of FIGS. 11A, 11B and 11C reveals that the only significantdifference between the Demux device 180 and the spectrophotometricmonitoring device 200 is that the output fiber array 184 of the Demuxdevice 180 is replaced in the monitoring device 200 by a photodetectorlinear array 202 that is positioned at the focal plane of thecollimating/focusing lens assembly 87. The monitoring device 200functions exactly as described for the Demux device 180 with theexception that in the monitoring device 200 the spatially separatedfocused wavelength channel beams 191 are incident on differentphotosensitive elements in the photodetector array 202 and, thereby,generate an independent electrical signal 204 for each wavelengthchannel. The amplitude of each electrical signal is proportional to theaverage light intensity of the wavelength channel beam incident on thephotodetector element generating that signal. While only threewavelength channels are shown being used with the monitoring device 200illustrated in FIGS. 11A, 11B and 11C, it is evident that this devicecan be used with many more wavelength channels. An InGaAs photodetectorarray will normally be incorporated into monitoring devices used forcommunication systems operating in the 1280 to 1620 nanometer spectrumregion. Commercial InGaAs photodetector arrays are available with 128,256, and 512 photodetector elements having either 25 or 50 micronspacing between element centers.

[0130] Comparison of the wavelength monitoring device depicted in FIGS.11A, 11B, and 11C with the corresponding devices depicted in FIGS. 5 and6 shows that, by using a dual pass transmission grating arrangementversus a single pass grating arrangement, one has spatially andfunctionally combined the collimating and focusing lens assemblies,significantly decreased the spatial separation between the input andoutput image planes of the device, and decreased the size of the device.It should be noted that current commercially available InGaAsphotodetector arrays used in these devices have overall package sizes inthe range of 63 millimeters by 25 millimeters, which is significantlylarger than what is depicted in FIGS. 11A, 11B and 11C when compared tothe other components depicted in these figures. Because of therelatively large size of current InGaAs photodetector arrays, there hasto be a significantly greater distance between the array unit and inputfiber element in FIG. 11B, which increases the requirements on theperformance of the collimating/focusing lens assembly used in thisdevice.

[0131] By separating the collimating and focusing lens functions in theFIGS. 5 and 6 monitoring devices, one can optimize the lenses used forthese imaging functions and thereby potentially improve upon thecost/performance ratio of the device, as previously described.

[0132] One can further increase the wavelength dispersion power of thedual pass grating arrangements of FIGS. 7, 8 and 9 by incorporating abeam expanding prism element into these devices as illustrated in FIG.12. Comparison of FIG. 12 with FIG. 9 reveals that the devices in thesefigures are essentially the same except that the grating substrate 12 indevice 160 of FIG. 9 is replaced in device 210 of FIG. 12 by the beamexpanding prism element 212 and that the grating 15 is now deposited onthe input optical transmitting surface of the glass block 152 in device210. With regard to the dual pass grating diffraction properties, thedevice 210 functions as described for the devices illustrated in FIGS.7, 8 and 9. As depicted in FIG. 12, a single collimated wavelength beam162 is incident on the device 210 at the Littrow diffraction conditionfor the device 210 arrangement and is retrodiffracted back along theincident beam 162 as beam 166. Referring to FIG. 12, it will be seenthat the prism element 212 expands the size of the incident beam 162prior to that beam being incident on the grating surface 15 and sinceprism 212 functions in a reversible manner it reduces the size of thebeam 214 rediffracted from grating 15.

[0133] The increase in the wavelength dispersion power of device 210,relative to that achieved with the devices of FIGS. 7, 8 and 9, isdetermined by how much the prism element 212 reduces the size of thebeam 166 exiting from the prism 212 relative to the size of the beam 214propagating within prism 212. It can be shown that the increasedwavelength dispersion power, Q, provided by the prism element 212 ofdevice 210 is given by:

Q=cos θ ₁/cos θ₂,  (5)

[0134] where θ₁ and θ₂ are, respectively, the angles that the beams 214and 166 make with respect to the normal to the surface 216 of prismelement 212. In the preferred 210 device embodiment, the Q value for theprism element 212 is in the range of about 1.3 to 2.0. The angularseparation between the wavelength channel beams exiting the device 210is given by the product Q dθ_(dS) where the value of dθ_(dS) iscalculated using Equation (4).

[0135] With the device 210 of FIG. 12, one can achieve a wavelengthdispersion power that is equal to an effective λ/D ratio value in therange of about 2.0 to 4.8 and still achieve essentially equaldiffraction efficiency values for S and P polarized optical components.One can achieve high radiometric throughput efficiency for the prismelement 212 for both S and P optical polarization by applyingantireflection coatings to the optical transmitting surfaces of theprism. As depicted in FIG. 12, the non-optical transmitting andreflecting surfaces of glass block 152 and prism element 212 have beencoated with an optical absorption coating 156 that is designed to absorbthe nondiffracted zeroth order beam energy and other scattered andreflected light which may occur within elements 152 and 212.

[0136] Associated with the beam size reducing property of prism element122 of device 210 is the additional benefit with regard to linearizingthe spacing of the spatially separated focused wavelength channel signalbeams at either the photodetector array element in a wavelengthmonitoring device or at the output fiber-optic array in a Demux device.The wavelength channels of a WDM fiber-optic system are separated by afixed frequency spacing, such as 200, 100 or 50 GigaHertz, but have awavelength spacing between adjacent wavelength channels that variesslightly as a function of the frequency (wavelength) of the wavelengthchannel. For example, the wavelength spacing between adjacent wavelengthchannels of a WDM fiber-optic system having a 100 GigaHertz frequencyspacing between channels is approximately 0.86, 0.80 and 0.78nanometers, respectively, for wavelength channels having wavelengths inthe range of 1611, 1552, and 1530 nanometers.

[0137] This slight variation in wavelength spacing between the adjacentwavelength channels in a WDM fiber-optic system produces a correspondingnon-equal spacing variation in the spatial separation between thefocused wavelength channel beams incident on the photodetector arrays inthe wavelength monitoring devices depicted in FIGS. 5, 6, 11A, 11B and11C and in the spatial separation between the focused wavelength channelbeams incident on the fiber-optic output array in the Demux devicedepicted in FIGS. 10A, 10B and 10C. Another factor that contributesslightly to the non-equal spacing of the spatially separated focusedwavelength channel beams in these wavelength selection devices is the1/cos θ_(d) term in Equation (2), which is used for calculating theangular separation between the diffracted wavelength beams in thesedevices. The prism element 122 of device 210 can be designed so that itlinearizes the diffracted angular spacing between the wavelength channelbeams exiting from the prism element 122 and, thereby, enables Demux andwavelength monitoring devices that incorporate device 210 to utilize alinear spacing between adjacent channels in their output arrays.

[0138] Given its relatively high radiometric throughput efficiency, itshigh effective λ/D value, and its linearizing properties, the device 210of FIG. 12 provides significant advantages for use in WDM fiber-opticcommunication systems having 100, 50 or 25 GigaHertz spacing betweenwavelength channels. Beam expanding prism elements are used to increasethe wavelength dispersion resolution of grating-based wavelength tunabledye laser systems, as shown in an article by F. J. Duarte, “Newton,Prisms, and the “Opticks” of Tunable Lasers,” Optics and Photonics News,May 2000.

[0139] Schematically illustrated in FIG. 13 is spectrophotometer device217A that is essentially identical to the spectrophotometer device 120of FIG. 6 with the expectation that the device 217A incorporates alinearizing prism element 218 that functions similar to the prismelement 122 of device 210 in FIG. 12. The prism element 218 differssomewhat from the prism element 122 in that the angles of the prism havebeen changed so that the diffracted beam from grating element 10 issubstantially perpendicular to the input surface of prism element 218.Due to this prism angle change, the prism element 218 is not attached tothe grating element 10, as was the case for the prism element 212 ofdevice 210. As depicted in FIG. 13, prism element 218 has the same beamsize reducing property as described for prism element 212 of device 210which increases the wavelength dispersion power of device 217A andprovides the benefit with regard to linearizing the spacing of thespatially separated focused wavelength channel signal beams at thephotodetector array 94 of device 217A. As also depicted in FIG. 13, thenon-optical transmitting surfaces of prism element 218 have been coatedwith an optical absorption coating that is designed to absorb anynon-diffracted zeroth order beam energy that might enter the prism andother scattered and reflected light which may occur within prism element218.

[0140] As depicted in FIG. 13, the prism element 218 reduces the size ofthe beam exiting prism element 218 by about 1.65 times compared to thebeam propagating within the prism element. The angular separationbetween the wavelength channel beams exiting prism element 218 isapproximately 1.65 dθ_(d) where dθ_(d) is the diffracted angularseparation between wavelength beams exiting the grating element 10 ascalculated by Equation (2). Prism element 218 increases the wavelengthdispersion power of device 217A by approximately 1.65 times thatachieved by device 120 of FIG. 6. Therefore, device 217A can achieve thesame spatial separation between focused wavelength channel beams at thephotodetector array 94 that device 120 achieves, but can achieve thisseparation using a focusing lens assembly 92 that has a focal lengththat is approximately 1.65 times shorter than the corresponding focallength used in device 120.

[0141] As previously noted, prism element 218 provides additionalbenefit with regard to linearizing the spacing of the spatiallyseparated focused wavelength channel beams at the photodetector array94. Analysis has shown that when the spectrophotometer device 120 ofFIG. 6 is used to monitor the signal of a WDM fiber-optic system havinga 100 GigaHertz frequency spacing between wavelength channels, thespatial separations between the focused wavelength channel beams at thephotodetector array 94 in device 120 are non-equally spaced and,therefore, do not match the 25 or 50 micron equally spaced intervalsbetween the photosensitive elements of commercial available InGaAsphotodetector linear arrays.

[0142] To illustrate the non-equal spacing error in the spatiallyseparated focused wavelength channel beams in device 120, it is assumedthat the spatial separated focused channel beams in device 120 areincident on adjacent photosensitive elements in the photodetector array94 and that these elements have a 50 micron spacing between elementcenters, that the device 120 incorporates a surface-relief transmissiongrating element 10 having a λ/D ratio of 1.0 for an optical wavelengthof 1550 nanometers, that θ_(i)=θ_(d)=30 degrees for a wavelength of 1550nanometers and that the focusing lens assembly 92 in this device has afocal length of approximately 84 millimeters. When this device 120configuration is used to monitor a WDM fiber-optic system beam having100 different wavelength channel signals each spaced by 100 GigaHertzand having a wavelength spectrum from about 1530 to 1612 nanometers,calculations indicate that the non-equal spacing error between theadjacent focused wavelength channel beams at the photodetector array 94accumulates and results in a total spacing error of approximately 49microns between the shortest and longest focused wavelength channelsignal beams. That is, if the photodetector array 94 in device 120 ispositioned so that the shortest wavelength channel signal beam isincident on the center of the first photosensitive element of thephotodetector array 94, the longest wavelength channel signal beam willland approximately 49 microns from the center of the hundredthphotosensitive element of the photodetector array 94, with progressivelyshorter wavelength channel signal beams having progressively smallerpositional errors with respect to the photosensitive element on whichthey are supposed to be incident on in the photodetector array 94. This49-micron positional error is not acceptable.

[0143] Calculations show that when spectrophotometer device 217A of FIG.13 is used to monitor a WDM fiber-optic system beam having 100 differentwavelength channel signals each spaced by 100 GigaHertz and having awavelength spectrum from about 1530 to 1612 nanometers, the spatiallyseparated focused wavelength channel signal beams at the photodetectorarray 94 have substantially the same 50 micron spacing between all thewavelength channel signal beams, with about a total 1 micron spacingerror between the shortest and longest wavelength channel signal beams,when the following configuration conditions are assumed for device 217A.Device 217A incorporates a surface-relief transmission grating having aλ/D ratio of 1.0 with θ_(i)=θ_(d)=30 degrees for an optical wavelengthof 1550 nanometers, the prism element 218 has an index of refraction ofapproximately 1.51 with θ₁ being approximately 35 degrees and θ₂ beingapproximately 60 degrees which corresponds to a beam size reduction byprism element 218 of approximately 1.65 times, and that the focusinglens assembly 92 has a focal length of approximately 50 millimeters. Themaximum total error spacing of approximately 1 micron between theshortest and longest wavelength channel signal beams at thephotodetector array 94 for this device configuration is very acceptablesince each photosensitive element in a photodetector array having a 50micron spacing between photosensitive elements has a photosensitive areawidth along the array equal to the element spacing of 50 microns and,therefore, a positional error in the range of 1 to 2 microns stillplaces the focused wavelength channel beam essentially in the middle ofthe photosensitive element.

[0144] Schematically illustrated in FIG. 14 is spectrophotometer device217B that is essentially identical to the spectrophotometer device 217Aof FIG. 13 with the expectation that the linearizing prism element 219of device 217B now incorporates the transmission surface-relief grating15. The prism element 219 performs the same functions as the combinationof the grating element 10 and prism element 218 of device 217A and,therefore, device 2177B functions as described for device 217A.

[0145] While there are advantages associated with combining thefunctions of the grating element 10 and prism element 218 of device 217Ainto the single prism element 219 of device 217B, the prism element 219does not provide as good results as the separate grating and prismelements provide with regard to passively athermalizing the performanceof the spectrophotometer device so that it meets operatingspecifications when used over the 70 degree centigrade temperature rangespecified for fiber-optic telecommunication applications without theneed for active control. The reason why the separate grating element 10and the separate prism element 218 provide better results than the dualfunctioning prism element 219 with regard to athermalizing deviceperformance is that the grating element achieves the bestathermalization performance when fabricated using a substrate materialhaving a low thermal expansion coefficient, such as fused silica or ULEglass, while the linearizing prism element achieves the bestathermalization performance when fabricated using a glass materialhaving a low thermal coefficient of refraction (dn/dT), such as BK7 andK5 glasses. Unfortunately, low thermal expansion glasses such as fusedsilica, ULE and Ohara Clearceram-Z have a thermal coefficient ofrefraction that is approximately 10 times larger than that achieved withK5 glass and about 5 times larger than that achieved with BK7 glass.Optical glasses such as BK7 and K5 have a thermal coefficient of thermalexpansion that is approximately 10 times larger than that achieved withfused silica and about 50 to 100 times greater than that achieved withULE or Ohara Clearceram-Z. Therefore, as the preceding discussionillustrates, better athermalization of device performance is achieved byusing different glass materials for the grating and prism elements whichdictates that they be fabricated as separate components to avoidthermal-induced stress associated with the optical bonding of materialshaving significantly different thermal expansion coefficients.

[0146] The doubling of the angular wavelength dispersion power of thetransmission grating elements in FIGS. 7, 8, 9 and 12 was accomplishedby reflecting the diffracted beam back through the grating element. Onecan achieve this doubling of grating dispersion power by cascading twoseparate transmission grating elements, that is, physically arrangingtwo surface-relief transmission grating elements so that a beamdiffracted by the first grating element undergoes diffraction by thesecond grating element. One could further increase the wavelengthdispersion power of transmission grating-based devices by cascadingmultiple grating elements. For example, one could achieve effectivelyfour times the wavelength dispersion power of a grating element byphysically cascading four individual transmission grating elements or byreflecting the diffracted beam back through two cascaded transmissiongrating elements. The only significant negative associated with thismultiple cascaded transmission grating technique is the radiometricefficiency loss associated with the multiple diffraction events.Surface-relief grating-based devices utilizing physically cascadedgrating elements are illustrated in FIGS. 15 through 24.

[0147] It should be noted that one of the potential advantages of thephysically cascaded grating arrangements in FIGS. 15 through 24 relativeto the dual pass grating arrangements in FIGS. 7 through 12, is thatdifferent λ/D ratio values can be used for the individual gratingelements used in the physically cascaded grating arrangements in FIGS.15 through 23. It should also be noted that the physically cascadedgratings in FIGS. 15 through 23 are arranged so that the wavelengthdispersion power of the device incorporating the gratings is essentiallyequal to the sum of the wavelength dispersion power of the individualgratings used in the device. The gratings in these devices are arrangedso that the beam diffracted from the first grating is incident on thesecond grating so that its angle of incidence is on the same relativeside of the normal to the second grating surface that the incident beammakes relative to the normal of the first grating surface. That is, asillustrated in FIGS. 15 through 23, the beam is always incident on theright side of the normal to the grating surfaces as viewed in the beampropagating direction. The cascaded grating arrangements in FIGS. 15through 23 are arranged so that the individual grating elements in thesearrangements are operated relatively close to the Littrow directioncondition.

[0148]FIG. 15 is a schematic of a dual pass cascaded grating-basedwavelength selection device 220 which is similar to the device 130depicted in FIG. 7 but differs therefrom in replacing the mirror element122 in device 130 with a surface-relief reflection diffraction gratingelement 222 in device 220. As will be apparent, this modificationsignificantly increases the wavelength dispersion power of the device220 relative to that of device 130 of FIG. 7. The incident beam 84 tothe transmission grating element 10 in device 220 contains λ₁ and λ₂wavelength components. After the incident beam 84 is diffracted by thegrating element 10, these optical wavelength components are angularlyseparated by Δθ. The grating element 222 is angularly orientated so thatthe λ₁ wavelength beam 132 is retrodiffracted back on itself, that is,the grating element 222 operates at the Littrow condition, θ_(i)=θ_(d),for the λ₁ wavelength beam 132. Because the grating element 10 functionsin a reversible manner, the grating element 10 rediffracts theretrodiffracted λ₁ wavelength beam 132 back along the direction of theincident beam 84 as beam 134. This dual pass transmission multi-gratingarrangement mimics a single reflection grating element operating at theLittrow condition for the λ₁ wavelength beam, since this beam isretrodiffracted back on itself After diffracting from the gratingelement 222, the λ₂ wavelength beam 136 propagates back to the gratingelement 10 where, due to its angle of incidence, the λ₂ wavelength beam136 is rediffracted from the grating element 10 with an angle equal toapproximately 3Δθ relative to the propagating direction of theretrodiffracted λ₁ wavelength beam 134 when the transmission gratingelement 10 and the reflection grating element 222 have approximately thesame λ/D ratio values.

[0149] As shown in Christopher Palmer's “Diffraction Grating Handbook,”supra, essentially equal diffraction efficiency values for S and Ppolarized optical components can be achieved for sinusoidalsurface-relief reflection gratings when their λ/D ratio is in the rangeof about 0.7 to 0.85. This reference also shows that surface-reliefreflection gratings having a triangular blazed grating line grooveprofile achieve essentially equal diffraction efficiency values for Sand P polarized optical components when these gratings have λ/D ratiovalues of between about 0.1 to 0.85. It is apparent from this referencethat approximately 0.85 is the largest λ/D ratio value that can be usedwith surface-relief reflection gratings and still achieve essentiallyequal diffraction efficiency values for S and P polarized opticalcomponents. Therefore, one may elect to use a surface-relief reflectiongrating element 222 in device 220 having a λ/D value of about 0.8 incombination with a surface-relief transmission grating element 10 thathas a λ/D value of between 0.8 and 1.2 and, thereby increase theeffective λ/D ratio value of device 220 while still achievingessentially equal diffraction efficiency values for S and P opticalpolarizations.

[0150] Using the dual pass multi-grating combination in device 220, asurface-relief grating-based device can be constructed with effectiveλ/D grating values of 2.3 to 3.2 while achieving essentially equaldiffraction efficiency values for S and P polarized optical components.The dual pass multi-grating combination device 220 achieves theseeffective high λ/D values by cascading the grating dispersion power ofthe grating elements in the device, that is, the effective λ/D value isthe sum of the λ/D values of the individual grating elements that thebeam is diffracted by. In FIG. 15 the beam undergoes three diffractionssince the beam is passed twice through grating element 10. Thiscascading of the grating dispersion power does not effect the wavelengthfilter function of the grating-based devices incorporating this cascadedgrating arrangement, since the wavelength filter function of thesedevices is determined by the physical dimensions of the output arraystructures used in those devices.

[0151] One can show that the angular separation, dθ_(dS), between thedifferent wavelength channel beams that undergo diffraction by cascadeddiffraction grating elements can accurately be approximated by using thesum of the angular separations that each grating element in the cascadedgrating arrangement introduces into the beams, that is:

dθ _(dS) =dθ _(d1) +dθ _(d2) +dθ _(d3) + . . . +dθ _(dn),  (6)

[0152] where dθ_(d1), dθ_(d2), dθ_(d3), and dθ_(dn) are, respectively,the individual angular separations, as calculated by Equation (2), thatthe different wavelength beams experience as they are diffracted by theindividual grating elements 1, 2, 3, and n of the cascaded gratingarrangement having n cascaded grating elements. For example, the dualpass grating devices in FIGS. 7, 8 and 9 have 2 cascaded gratingelements and, therefore, according to Equation (6)dθ_(s)=dθ_(d1)+dθ_(d2)=2dθ_(d), as previously calculated with Equation(4). For the device 220 in FIG. 15,dθ_(s)=dθ_(d1)+dθ_(d2)+dθ_(d3)=2dθ_(d)+dθ_(d2), where dθ_(d) is theangular separation for the transmission grating element 10 and dθ_(d2)is the angular separation for the reflection grating element 222.

[0153] Referring again to FIG. 15, one can change the wavelength of thebeam 134 retrodiffracted back on itself and, thus change the wavelengthtuning parameters of the device 220, by rotating the grating element 222in the direction of arrow 138 and/or arrow 140 by conventional means, aswas also described in regards to FIG. 7.

[0154] As illustrated in FIG. 16, one can configure the device 220 ofFIG. 15 using a solid glass block element 232 that incorporates asurface-relief transmission grating 15 and a surface-relief reflectiongrating element 222 that is attached to the output optical transmittingsurface of glass block 232. The dual pass multi-grating device 230 ofFIG. 16 functions as described for the device 220 of FIG. 15. Asdepicted in FIG. 16, a single collimated wavelength beam 162 is incidenton the dual pass multi-grating device 230 at the Littrow diffractioncondition for the device 230 arrangement and is retrodiffracted backalong the incident beam 162 as beam 166. A sealing element 168, such asepoxy, is used in device 230 to encapsulate the air gap layer 170 thatexists between the surface-relief reflection grating surface of element222 and the output optical transmitting surface of the glass blockelement 232. The main function of the sealing element 168 is to preventcontaminants, liquids or solvent vapors that could damage the gratingsurface from entering the air gap layer 170; and, as illustrated in theExamples, not every sealing element will function well in this device.The output optical transmitting surface of the glass block 152 has to beantireflection coated to minimize optical reflection losses at thatsurface. As depicted in FIG. 16, the non-optical transmitting surfacesof the glass block 152 have been coated with an optical absorptioncoating 156 that is designed to absorb the nondiffracted zeroth orderbeam energy and other scattered light which may occur within the glassblock element 152.

[0155]FIG. 17 is a schematic view of a dual pass multi-grating device240 similar to the device 230 depicted in FIG. 16 but differingtherefrom in that the transmission grating surface 15 in device 240 isencapsulated between the substrate 12 of grating element 10 and theinput optical transmitting surface of glass block element 232 to whichelement 10 is attached, similar to manner shown in the device 160 ofFIG. 9. Furthermore, the surface-relief reflection grating element 222is directly attached (by, e.g., adhesive means, such as optical cement)to the glass block 232. The device 240 functions exactly as describedfor the device 230 of FIG. 16, except that the surface-relief reflectinggrating surface of element 222 of device 240 is immersed in the opticalcement used to optically bond element 222 to element 232. Under theseimmersed grating conditions, the effective λ/D of the grating element222 is reduced by the index of refraction of the optical cement used tobond the grating element 222 to the glass block 232. One can compensatefor the reduction in the λ/D of the grating element 222 as a result ofbeing immersed in a media having an index of refraction larger than the1.0 value for air by starting with a grating element 222 that has ahigher λ/D value. Typically one starts with a λ/D value that is n timeslarger than the effective the λ/D value that one wants to achieve forthe immersed grating element, where n is the refractive index of thecement used to bond element 222 to element 232. Most optical cementshave a refractive index in the range of about 1.45 to 1.6. For example,if one wants to have an immersed sinusoidal surface-relief reflectiongrating element 222 that has an effective λ/D value of about 0.8 and,thereby, achieve essentially equal diffraction efficiency values for Sand P polarized optical components, one would start with a gratingelement having a λ/D value of about 1.2, assuming that the opticalcement used to bond element 22 to element 232 in device 240 had an indexof refection of 1.50.

[0156]FIG. 18 illustrates how one can achieve the doubling of gratingdispersion power by physically cascading two surface-relief transmissiongratings 15 and 15′, i.e., physically arranging two transmission gratingelements so that a beam diffracted by the first grating undergoesdiffraction by the second grating. As depicted in FIG. 18, the twogratings 15 and 15′ are deposited onto the input and output opticaltransmitting surfaces of the glass block element 252. The singlewavelength collimated incident beam 162 is diffracted by the firstgrating 15 as beam 254. The collimated diffracted beam 254 propagates inthe glass block element 252 and is incident upon the second diffractiongrating 15′, which diffracts the beam 254 as collimated beam 256. Thenon-optical transmitting surfaces of the glass block 252 are coated withan optical absorption coating 156 that is designed to absorb thenondiffracted zeroth order beam energy and other scattered light thatmay occur within the glass block element 252.

[0157] The wavelength dispersion power of the dual cascaded transmissiongrating element 250 is essentially the sum of the wavelength dispersionpowers of each of the gratings 15 and 15′. That is, the device 250 hasan effective λ/D value which is equal to the sum of the λ/D values forthe individual grating elements 15 and 15′ used in the device. One canachieve essentially equal diffraction efficiency values for S and Ppolarized optical components for the element 250 by using surface-relieftransmission gratings 15 and 15′ that each have a λ/D ratio in the rangeof about 0.8 to 1.2, as shown by the data in FIG. 4. Using this λ/Dvalue range for gratings 15 and 15′, the element 250 can have aneffective λ/D ratio value of about 1.6 to 2.4 and achieve essentiallyequal diffraction efficiency values for the S and P optical polarizationcomponents. The angular separation between wavelength beams for element260 is calculated with Equation (6) as dθ_(dS)=dθ_(d1)+dθ_(d2), wheredθ_(d1) and dθ_(d2) are, respectively, the angular separation forgratings 15 and 15′, as calculated by Equation (2).

[0158] The spectrophotometer device 260 of FIG. 19 is essentiallyidentical to the spectrophotometer device 217A of FIG. 13 with theexception that diffraction grating element 10 of device 217A is replacedin device 260 with the dual cascaded transmission grating element 250 ofFIG. 18. Also, the linearizing prism element 218 and the beam foldmirror element 122 of device 217A have been combined into the singleprism element 262 in device 260 that performs the dual functions of beamfold mirror and linearizing prism element. The dual functioning prismelement 262 can achieve good athermalization performance sincefabricating it with a glass material having a low thermal coefficient ofrefraction does not affect the beam folding mirror function of theelement. Because the grating element 10 in FIG. 13 is replaced withelement 250 in device 260, device 260 effectively has approximatelytwice the wavelength dispersion power as that achieved with device 217A.The higher wavelength dispersion power of the device 260 enables thisdevice to utilize a shorter focal length for the focusing lens assembly92 in the device and/or the device can be used in WDM fiber-opticcommunication systems having finer wavelength spacing between wavelengthchannel signals. The ability to work with WDM systems having finerspacing between their wavelength channel signals is becoming moreimportant since the space between wavelength channels in fiber-opticcommunication systems is continuing to decrease.

[0159]FIG. 20 illustrates a dual cascaded transmission grating device270 which is similar to the device 250 of FIG. 18 but differs from thatdevice by using surface-relief transmission grating elements 10 and 10′that are, respectively, attached to the input and output opticaltransmitting surfaces of glass block 252. The grating surfaces ofelements 10 and 10′ are encapsulated using sealing element 168 insubstantial accordance with the method used to encapsulate the gratingsurface of element 10 in device 160 of FIG. 9. As illustrated in theexamples, not every sealing element will function well in this device.The device 270 functions as described for the device 250 of FIG. 18.

[0160]FIG. 21 illustrates a dual pass multi-grating device 280 that issimilar to the device 130 of FIG. 7 but replaces the diffraction gratingelement 10 of device 130 with dual cascaded grating device 270 of FIG.20. As depicted in FIG. 21, a single collimated wavelength beam 162 isincident on the device 280 at the Littrow diffraction condition for thedevice 280 arrangement and is retrodiffracted back along the incidentbeam 162 as beam 166. Device 280 functions essentially as described fordevice 130 of FIG. 7 with the exception that the wavelength dispersionpower of the device 280 is substantially two times as great as thatachieved with device 130 for the case where the grating elements 10 and10′ of device 280 have the same λ/D value as the grating element 10 indevice 130. The device 280 can have an effective λ/D ratio of about 3.2to 4.8 and still achieve essentially equal diffraction efficiency valuesfor S and P polarized optical components by using grating elements 10and 10′ that each have λ/D values in the range of about 0.8 to 1.2. Theangular separation between wavelength beams for device 280 is calculatedwith Equation (6) as dθ_(dS)=2dθ_(d1)+2dθ_(d2), where dθ_(d1) anddθ_(d2) are, respectively, the angular separation for grating elements10 and 10′, as calculated by Equation (2).

[0161]FIG. 22 illustrates a dual pass multi-grating device 290 that issimilar to the device 220 of FIG. 15 but replaces the diffractiongrating element 10 of device 220 with dual cascaded grating device 270of FIG. 20. As depicted in FIG. 20, a single collimated wavelength beam162 is incident on the device 290 at the Littrow diffraction conditionfor the device 290 arrangement and is retrodiffracted back along theincident beam 162 and beam 166. Device 290 functions essentially asdescribed for device 220 of FIG. 15 with the exception that thedispersion wavelength power of the device 220 is approximately 1.67times greater than that achieved for the device 220 for the case wheredevice 220 and device 290 use grating elements having essentially thesame λ/D values. The device 290 can have an effective λ/D ratio of about5.0 to 8.8 and still achieve essentially equal diffraction efficiencyvalues for S and P polarized optical components when the surface-relieftransmission grating elements used in the device each have λ/D values ofabout 0.8 to 1.2 and the surface-relief reflection grating element 222has a λ/D value of about 0.7 to 0.85. The angular separation betweenwavelength beams for device 290 is calculated with Equation (6) asdθ_(dS)=2dθd1+2dθ_(d2)+dθ₃, where dθ_(d1), dθ_(d2) and dθ_(d3) are,respectively, the angular separation for grating elements 10, 10′ and222, as calculated with Equation (2).

[0162] The transmission multi-grating device 300 depicted in FIG. 23 issimilar to the device 270 in FIG. 20, with the exception that thedispersion power has been further increased by stacking a thirddiffraction grating element 10″ to the grating elements 10 and 10′ thatare incorporated in device 270 of FIG. 20. Device 300 functions asdescribed for the device 270 of FIG. 20 with the exception that, indevice 300, the collimated beam 256 diffracted from grating element 10′propagates in the glass block 252′ to grating element 10″ where it isdiffracted as collimated beam 302. The effective λ/D ratio value for thedevice 300 is essentially equal to the sum of the λ/D values for theindividual grating elements 10, 10′ and 10″. Therefore, device 300 canbe fabricated with an effective λ/D ratio value of about 2.4 to 3.6while still achieve essentially equal diffraction efficiency values forS and P polarized optical components by using surface-relieftransmission grating elements for gratings 10, 10′ and 10″ that eachhave λ/D values in the range of about 0.8 to 1.2. The angular separationbetween wavelength beams for device 300 is calculated with Equation (6)as dθ_(dS)=dθ_(d1)+dθ_(d2)+dθ_(d3), where dθ_(d1), dθ_(d2), and dθ_(d3)are, respectively, the angular separation for grating elements 10, 10′and 10″, as calculated with Equation (2).

[0163] In device 300 the individual grating elements 10, 10′ and 10″ areattached to the corresponding optical transmitting surfaces of glassblock elements 252 and 252′ using sealing elements 168. Sealing elements168 encapsulate the grating surfaces of elements 10, 10′ and 10″ insubstantial accordance with the method used to encapsulate the gratingsurface of element 10 in device 160 of FIG. 9. As depicted in FIG. 23,grating element 10′ is optically attached to glass block 252′. One coulduse optical cement to bond grating element 10′ to glass block 252′ orone could directly optically contact grating element 10′ to glass block252′.

[0164] As depicted in FIG. 23, the substrate material of grating element10′ and the glass block 252′ have essentially the same index ofrefraction and, therefore, the beam 256 propagates from grating element10′ into glass block 252′ as if the combination of elements 10′ and 252′were fabricated from a single continuous glass block element for thecase where these elements are either optically contacted together orbonded using an optical cement that has an index of refraction that isessentially equal to that of the refractive indices used for elements10′ and 252′. For the case where the grating element 10′ and glass blockelement 252′ have different indices of refraction and/or the opticalcement used to bond elements 10′ and 252′ together has a different indexof refraction relative to the indices of refraction for elements 10′ and252′, the beam 256 propagates from grating element 10′ into glass blockelement 252′ but some of its intensity is lost due to the reflectionloss that occurs at the interface boundary surface between materialsthat have different indices of refraction. These reflection losses areless than 0.5 percent per interface boundary surface when the indices ofrefraction for elements 10′ and 252′ and the optical cement used to bondthem together are relatively close, that is, when the difference inthese indices of refraction are less than approximately 0.2 formaterials having an index of refraction in the range of about 1.40 to1.70.

[0165] The optical bonding of grating element 10′ to glass block element252′ in device 300 of FIG. 23, in effect, creates a surface-relieftransmission grating surface on the input optical transmitting surfaceof block element 252′. While this technique for creating a gratingsurface on the optical transmitting surface of a glass block element isonly illustrated in FIG. 23, it can be used to create the transmissiongrating surface 15 on the input optical transmitting surface of theglass blocks in FIGS. 8, 12, 14 and 16 and the grating surfaces 15 and15′ on the input and output optical transmitting surfaces of glass block252 in FIG. 18. This method of creating a surface grating 15 on theinput and/or output transmitting surfaces of a glass block by opticallybonding a grating element 10 to the surface of the glass block elementprovides significant advantages with regard to manufacturing the gratingsurfaces 15 on the input optical transmitting surfaces of the glassblocks in FIGS. 8, 12, 14 and 16 and the grating surfaces 15 and 15′ onthe input and output optical transmitting surfaces of the glass block inFIG. 18.

[0166] It is much easier to create a surface-relief photoresist gratingon a parallel plate substrate element than to create a surface-reliefphotoresist grating on a non-parallel shaped glass block element. Also,multiple surface-relief photoresist grating elements can be fabricatedon a single large substrate element, in the same manner that multipleintegrated circuit elements are fabricated on a single silicon wafer. Alarge substrate containing multiple grating elements can be cut up toyield smaller grating elements having a size suitable for the devicesthat they will be used with. These grating elements cut from the largersubstrate element can be used as a stand alone element as illustrated inFIGS. 5, 6, 7 and 15, attached to a glass block element with an airspacing layer between the grating surface and the optical transmittingsurface of the glass block element as illustrated in FIGS. 9, 16, 17,20, 21 and 23, or optically bonded to a glass element as illustrated inFIG. 23, and, while not specifically illustrated, used to create thegrating surface on the glass block elements incorporated in the devicesillustrated in FIGS. 8, 10A, 10B, 11A, 11B, 12, 14, 16, and 17.

[0167] One can further increase the wavelength dispersion power of thetransmission multi-grating device 300 of FIG. 23 by either addinganother transmission grating element to the device, incorporating a beamfold mirror in the device that retroreflects the diffracted beam 302back through the device or by incorporating a reflecting grating elementin the device that retrodiffracts the diffracted beam 302 back throughthe device. It is anticipated that the wavelength dispersion power ofthe devices shown in this specification are suitable for both presentand future grating-based devices used in fiber-optic communicationsystems.

[0168]FIG. 24 illustrates a dual cascaded transmission grating device310 which is similar to the device 250 of FIG. 18. Device 310 functionsessentially as described for device 250 with the exception that the beam254 diffracted from grating 15 undergoes two reflections within theglass block element in device 310 before being incident on grating 15′which diffracts it as beam 256. As depicted in FIG. 24, the glass blockelement of device 310 is composed of two glass block elements 312 and314 that are optically bonded together. Glass block elements 312 and 314are fabricated from the same type of glass material and are bondedtogether by being either directly optically contacted or by using anoptical cement that has an index of refraction that is essentially equalto the refractive index of glass block elements 312 and 314 and, therebyforms essentially a single continuous glass block element. The blockelement 312 is a roof prism element having a 90-degree angle between itsreflective mirror coated surfaces 154 and 154′. The block element 314 isa prism element having a base length that matches the length of thehypotenuse leg of block element 312 while the enclosed angle between theother two leg surfaces of element 314 is chosen to facilitate thediffraction angle conditions of device 310.

[0169] Device 310 is included in this specification to illustrate thatbeam folding mirror surfaces can be incorporated into the cascadedmulti-grating devices presented in this specification and, thereby,change the angular propagation path that the beam undergoes with thedevice and the angular direction of the beam exiting the device relativeto the incident beam direction. Comparison of device 310 in FIG. 24 withdevice 250 of FIG. 18 shows that the inclusion of beam folding mirrorsurfaces as implemented in the embodiment in FIG. 24 does notessentially affect the wavelength dispersion power of the device.

[0170] One of the most fundamental operations in a communication networkis the selective switching (add/drop) of signals between differenttransmission paths of the network. A number of techniques have beendemonstrated for building optically based add/drop wavelength multiplex(ADWM) devices that optically switch different wavelength channelsbetween different fiber ports in a WDM fiber-optic communication system.U.S. Pat. No. 5,960,133 discloses methods for building ADWM devices thatutilize reflection grating elements to perform the wavelength channelselection function in these devices. The entire disclosure of thisUnited States patent is hereby incorporated by reference into thisspecification.

[0171] Schematically illustrated in FIGS. 25A and 25B is an ADWM device320 that is similar to the device 19 in FIG. 2 of U.S. Pat. No.5,960,133, but differs from that device in that device 320 uses dualcascaded transmission grating element 270 of FIG. 20 in place of thereflection grating element used in the FIG. 2 device 19 of U.S. Pat. No.5,960,133. Because the elements used to fabricate the input and outputports and the micro electromechanical (MEM) mirror switching elements ofthe ADWM devices disclosed in U.S. Pat. No. 5,960,133 are relativelylarge, these ADWM devices can benefit from the flexibility of elementplacement provided by replacing the reflection grating elements used inthe ADWM devices of U.S. Pat. No. 5,960,133 with surface-relieftransmission grating-based elements, in the same manner that thewavelength channel monitoring devices of FIGS. 5 and 6 benefit from theuse of transmission grating element 10. These ADWM devices can alsobenefit from the increased wavelength dispersion power provided by thecascaded transmission grating arrangements presented in thisspecification, as illustrated by device 320 in FIGS. 25A and 25B.

[0172] With reference to FIG. 25A, ports P1 and P2 provide generallyparallel but separate optical beams 322 and 324 that are incident to thedual cascaded grating element 270. Beams 322 and 324 contain λ₁ andλ_(2 wavelength channel signals. After diffraction from element 270 the incident beams 322 and 324 are separated into their respective wavelength components. The λ)₁ wavelength components of beams 322 and 324 are, respectively, 326 and326′ and are depicted in FIG. 25A as solid lines while the λ₂ wavelengthcomponents of beams 322 and 324 are, respectively, 328 and 328′ and aredepicted as dashed lines in FIG. 25A. As illustrated in FIG. 25A, thediffracted beams having different wavelengths are angularly separatedwhile those of the same wavelength remain substantially parallel. A lens334 focuses all of the beams from element 270 onto a micro-mirror array336 comprising separately tiltable micro-mirror elements 338 and 340.

[0173] In the first position of the micro-mirror elements 338 and 340,illustrated in FIG. 25A by the solid lines, the mirror elements 338 and340 reflect both wavelength beams received from port P1 directly back toport P1 as beam 330. That is, in this first position the mirrors areorientated perpendicular to the beams 326 and 328. For this first mirrorposition no wavelength signal channels are either added to or droppedfrom port P1 of the device 320. However, when mirror elements 338 and340 are in the second position, illustrated by the dotted lines in FIG.25A, the mirror elements 338 and 340 reflect wavelength beams receivedfrom port P1 to port P2. That is, in the second position the mirrorelement 338 is orientated perpendicular to the bisector of the beams 326and 326′ and the mirror element 340 is orientated perpendicular to thebisector of the beams 328 and 328′. In the second position, the mirrorelements 338 and 340 also reflect wavelength beams received from port P2to port P1. For this second mirror position both the λ₁ and λ₂wavelength channel signals of beam 322 are dropped from port 1 and addedto port 2 while both the λ₁ and λ₂ wavelength channel signals of beam324 are dropped from port 2 and added to port 1.

[0174]FIG. 25B illustrates the case where mirror element 338 of device320 is orientated in the first position, same as illustrated in FIG.25B, while mirror element 340 is orientated in the second position. Forthe mirror orientation arrangement in FIG. 25B, the λ₁ wavelength beamfrom port 1 is reflected back to port 1 and comprises part of beam 330′while the λ₂ wavelength beam from port 1 is reflected to port 2 as beam332 and the λ₂ wavelength beam from port 2 is reflected to port 1 andcomprises part of beam 330′. For the mirror arrangement in FIG. 25B, theλ₂ wavelength channel signal of beam 322 is dropped from port 1 ofdevice 320 and added to port 2 of the device while the λ₂ wavelengthchannel signal of beam 324 from port 2 is added to port 1 of the device.While only two wavelength channel signals and only two micro-mirrorelements are depicted in device 320 of FIGS. 25A and 25B, it is evidentthat device 320 can be fabricated with a micro-mirror array 336 having alarge number of micro-mirror elements and, thereby enable device 320 tobe used to add/drop a large number of wavelength channel signals.

[0175] As stated in U.S. Pat. No. 5,690,133, the device 19 of FIG. 2 ofthat patent, that is similar to that of device 320 of FIGS. 25A and 25B,has many desirable characteristics but suffers from some problems. Oneproblem being that optical circulator devices have to be connected tothe ports P1 and P2 to separate wavelength beam signals going inopposite directions. Optical circulator devices are expensive and addoptical insertion loss to the ADWM device. A further problem with thedevice 320 is that it cannot simultaneously direct the λ₁ wavelengthbeam from port P1 to port P1 while directing the λ₁ wavelength beam fromport P2 to port P2, or perform this same simultaneous switching functionfor the same wavelength for any of the other wavelength channel beams inthe device.

[0176]FIG. 5 in U.S. Pat. No. 5,960,133 shows what is claimed as animproved micro-mirror based add/drop device relative to the device 19 ofFIG. 2 of this patent. Schematically illustrated in FIG. 26 is an ADWMdevice 350 that is similar to the FIG. 5 device in U.S. Pat. No.5,960,133, but differs from that device in that device 350 uses dualcascaded transmission grating element 270 in place of the reflectiongrating element used in the FIG. 5 device of U.S. Pat. No. 5,960,133.The device 350 functions similar to that stated for device 320 of FIGS.25A and 25B, with the exception that device 350 uses four parallel,direction, input and output beam paths 352, 354, 356, and 358 arrangedin a two-dimensional array. The four beams in this arrangement are theinput beam 352, the output beam 354, the add beam 356, and the drop beam358. The input and add beams 352 and 356 propagate oppositely from theoutput and drop beams 354 and 358.

[0177] As explained in U.S. Pat. No. 5,960,133, the incorporation of thefour parallel beam paths 352, 354, 356, and 358 into device 270 enablesthis add/drop device to function without the need for optical circulatordevices and enables the device 350 to simultaneously switch a λ₁wavelength channel signal from the input beam path 352 to the drop beampath 358 while switching a λ₁ wavelength channel signal from the addbeam path 356 to the output beam path 354.

[0178] Replacement of the reflection grating elements in the FIG. 2 andFIG. 5 devices of U.S. Pat. No. 5,960,133 with a surface-relieftransmission grating or a dual cascaded transmission grating element, asillustrated in FIGS. 25A, 25B and 26, does not change the basic add/dropfunctions of these devices but improves device layout configurationwhile providing increased wavelength dispersion power, which becomesincreasingly important as the wavelength spacing in WDM fiber-opticsystems decreases.

[0179] A schematic side view is illustrated in FIG. 27A of a Mux/Demuxdevice 360 that is similar to the device shown in FIG. 1 of a paper byS. Bourzeix, et al. entitled “Athermalized DWDMMultiplexer/Demultiplexer,” (2000 National Fiber Optic EngineersConference Technical Proceedings, Vol. 2, pages 317-320), but differsprimarily from that device in that device 360 uses surface-relieftransmission grating element 10 in place of the surface-reliefreflection grating element used in the FIG. 1 device of the Bourzeix, etal. paper. The use of transmission grating element 10 in device 360facilitates the placement of the dihedral retroreflecting mirror element374 in relation to the grating element 10 while enabling the grating tooperate closer to the Littrow diffraction condition, relative to thatachieved when a reflection grating element is incorporated into thedevice. Also, the use of transmission grating element 10 of FIG. 27Aenables the dihedral mirror element 374 of this figure to beincorporated into a glass block element that includes the transmissiongrating, similar to the arrangement illustrated in FIGS. 8 and 9. It ismuch more difficult to incorporate the dihedral mirror element into aglass block element that includes the grating when a reflection gratingis used in the device.

[0180] As depicted in FIG. 27A, input optical wavelength channel signalinformation is delivered to device 360 by transmission fiber 182. Inputfiber 182 and output fibers 362 are held and spatially positionedrelative to each other and the other optical components in device 360 bythe fiber-optic array element 364. The optical beam emerging from theend of input fiber 182 is incident on a lens element (not shown) in themicrolens array 366. Microlens array 366 reduces the divergence angle ofthe beam from input fiber 182 by about 1 degree, which increases therelative channel width of the device. Beam 367 from the microlens array366 is collimated by lens 368. The collimated beam 367 from lens 368propagates through the birefringent crystal element 370, through thehalfwave retardation plate 372, to the grating element 10 where the beamis diffracted to the dihedral mirror element 374, the retroreflectedbeam from mirror element 374 propagates back to grating element 10 whereit is rediffracted and propagates back through the halfwave retardationplate 372 and birefringent crystal element 370 to lens 368. Theconverging beam 376 from lens 368 is incident on microlens array 366,which focuses the angularly separated wavelength channel beams of beam376 onto their corresponding output fibers 362 held in the fiber-opticarray element 364.

[0181] A schematic top view in FIG. 27B of a portion of the device 360more clearly illustrates how the birefringent crystal element 370,halfwave retardation plate 372 and dihedral mirror element 374collectively function together to control the polarization direction ofthe optical beam incident on grating element 10 and, thereby, enable thedevice 360 to achieve radiometric throughput efficiency values for S andP polarizations that are equal to within about 5 percent of each other.As depicted in FIG. 27A, the incident beam to grating element 10 and thediffracted beam from grating element 10 both make an angle of about 45degrees with regard to the normal to the surfaces of element 10.Therefore, the grating element 10 in device 360 has a λ/D ratio value ofapproximately 1.4142, which according to the data in FIG. 4 results inthe S polarized optical beam having almost 100 percent diffractionefficiency while the P polarized beam has almost zero percentdiffraction efficiency. For the configuration depicted in device 360,essentially only the S polarized optical component is diffracted fromgrating element 10 and, therefore, the other optical elementscollectively function together to ensure that only a S polarized beam isincident on the grating element 10.

[0182] With reference to FIG. 27B, the incident beam 367 to thebirefringent crystal element 370 is composed of both S and P polarizedoptical components where the P component 378 is depicted as an ellipsewith a dot at its center while the S component 380 is depicted as a boldarrow figure. Only the S and P polarization components to the left ofelement 370 in FIG. 27B are labeled with their respective numbers 380and 378. When beam 367 propagates through the birefringent crystalelement 370 its S and P polarized optical beam components propagate atan angle with respect to each other. As illustrated in FIG. 27B, the Ppolarized beam component of beam 367 propagates essentially straightthrough element 370 while the S polarized beam component of beam 367 isrefracted at an angle relative to the P polarization beam direction asit propagates through element 370. The length of the birefringentcrystal element 370 is chosen so that the P polarized beam path 382exiting the element 370 is spatially separated from the S polarized beampath 384 exiting the element 370, as illustrated in FIG. 27B.

[0183] The beam paths 382 and 384 are parallel and spatially separatedas they propagate through grating element 10 and dihedral mirror element374. As illustrated in FIG. 27B, the dihedral mirror element 374 has a90 degree angle between its reflecting mirror surfaces and, thereby,functions as a retroreflecting mirror element that redirects the beampropagating from element 10 to element 374 along beam path 382 topropagate back to element 10 along beam path 384 while redirecting thebeam that propagates from element 10 to element 374 along beam path 384to propagate back to element 10 along beam path 382. Positioned in beampath 382, but not in beam path 384, is halfwave retardation plate 372that converts the polarization direction of the oppositely propagatingbeams in beam path 382 from P polarization to S polarization for thebeam propagating from element 370 to element 10 and from S polarizationto P polarization for the beam propagating from element 10 to element370. The birefringent crystal element 370 functions in a reversiblemanner and, thereby, recombines the beams propagating in beam paths 382and 384 that are incident to element 370 into a single beam 376 thatpropagates from element 370 to lens 368.

[0184] For the optical arrangement illustrated in FIG. 27B, the beamspropagating in either direction of beam paths 282 or 284 that areincident on grating element 10 are S polarized and, therefore, haveequal diffraction efficiency values which enables the device 360 toachieve radiometric throughput efficiency values for S and P polarizedoptical components that are equal to within about 5 percent of eachother. The diffraction grating and dihedral mirror arrangement in device360 functions as a dual pass cascaded grating arrangement, as describedfor the dual pass grating device 130 of FIG. 7. The wavelengthdispersion power of device 360 is equal to approximately twice the valueof the wavelength dispersion power of grating element 10 used in thedevice. The angular separation between the different wavelength channelbeams exiting element 370 of device 360 are calculated using Equation(4). The λ/D value for the grating element 10 in device 360 iseffectively doubled. As shown by data presented in Pages 179-182 of theLoewen text discussed elsewhere in this specification, one can achievegreater than 70 percent diffraction efficiency for S polarization forsurface-relief transmission gratings having λ/D ratio values of about0.8 to approximately 2.0. Therefore, device 360 can be fabricated withan effective λ/D ratio value of about 1.6 to 4.0 and still achieveessentially equal radiometric throughput efficiency for values for S andP polarized optical components, that is, having values within about 5percent of each other.

[0185] A schematic side view is illustrated in FIG. 28 of a grating andmirror arrangement that enables the device 390 of this figure to achieveessentially equal radiometric throughput efficiency values for S and Poptical polarization components while using a surface-relieftransmission grating element 10 having a λ/D value of about 1.4142. Asdepicted in FIG. 28, the incident beam 367 to device 390 is composed ofboth S and P polarized optical components where the S component 378 isdepicted as an ellipse with a dot at its center while the P component380 is depicted as a bold arrow. Only the S and P polarized componentsto the left of grating element 10 are labeled with their respectivenumbers 378 and 380. Though it may appear that the polarizationdirection convention used in FIG. 28 is opposite to that used in FIG.27B, they are the same since FIG. 28 provides a side view relative tograting element 10 while FIG. 27B provides a top view relative tograting element 10.

[0186] Incident beam 367 of device 390 makes an angle of 45 degrees withrespect to the normal of the surface of grating element 10 and,therefore, if it is assumed that grating element 10 of device 390 has aλ/D value of about 1.4142 and a grating aspect ratio in the range of 1.3to 2.0, then according to the data in FIG. 4 the following diffractionconditions occur, which are depicted in FIG. 28. Grating element 10diffracts essentially 100 percent of the S polarized beam component ofthe incident beam 367 while passing through undiffracted essentially 100percent of the P polarized beam component of beam 367. For thesediffraction conditions, grating element 10 performs the same function asthe birefringent crystal element 370 of FIGS. 27A and 27B in thatgrating element 10 functions as a polarization beam splitter element.Combining the polarization beam splitter function into grating element10 improves device performance with regard to optical insertion loss andwavefront errors, as well as device cost, relative to the device 360arrangement in FIGS. 27A and 27B which incorporates a polarization beamsplitter element based on a birefringent crystal component. Also,grating element 10 in device 390 can have essentially half the widthused for the grating element in device 360 in FIGS. 27A and 27B sincethe beam propagating through element 10 in device 390 are collinearversus the spatial separated arrangement in device 360.

[0187] Both the diffracted and undiffracted beams in device 390 make anangle of 45 degrees to the normal to the grating surface of element 10.The S polarized diffracted beam propagates along beam path 382 of device390 until beam fold mirror element 122 redirects it along beam path 386in a counterclockwise direction. The P polarized undiffracted beampropagates along beam path 384 of device 390 until beam fold mirrorelement 122′ redirects it along beam path 386 in a clockwise direction.Halfwave retardation plate 372 positioned in beam path 386 converts thepolarization direction of the oppositely propagating beams in beam path386 from P polarization to S polarization for the clockwise propagatingbeam and from S polarization to P polarization for the counterclockwisepropagating beam. The counterclockwise propagating P polarized beam inbeam path 386 is redirected by beam fold mirror element 122′ along beampath 384 while the clockwise propagating S polarized beam in beam path386 is redirected by beam fold mirror element 122 along beam path 382.

[0188] Grating element 10 functions in a reversible manner and,therefore, element 10 diffracts essentially 100 percent of the clockwisepropagating S polarized beam in beam path 382 while passing throughundiffracted essentially 100 percent of the counterclockwise propagatingP polarized beam in beam path 384. Both of these diffracted andundiffracted beams propagate back along the incident beam path 367 asbeam 367. Because grating element 10 functions in a reversible manner,the S and P polarized optical components of beam 367 have essentiallyequal values, thereby enabling device 390 to have essentially equalradiometric efficiency values for S and P polarizations.

[0189] It should be noted that while the S and P polarized components ofthe incident beam 367 pass twice through grating element 10 of device390, each of these polarization components is only diffracted once byelement 10 and, therefore, device 390 has a wavelength dispersion poweras measured by its effective λ/D value that is just equal to that ofgrating element 10, which for the example depicted in FIG. 28corresponds to a λ/D value of about 1.4142. As shown by the data onPages 179-182 of the Loewen text discussed elsewhere in thisspecification, the polarization beam splitter function depicted forgrating element 10 in device 390 can be achieved for surface-relieftransmission gratings having a λ/D value of about 1.4142 toapproximately 2.0 and, therefore, the effective λ/D value for device 390can be from about 1.4142 to approximately 20 while achieving essentiallyequal radiometric throughput efficiency values for the S and Ppolarization components.

[0190] A major objective when designing grating-based devices forfiber-optic communication system applications is to incorporatetechniques in the design for passively athermalizing the performance ofthe devices so that they meet operating specifications when used overthe 70 degree centigrade temperature range specified for fiber-optictelecommunication applications without the need for active control. Oneof the major factors in these design techniques is to fabricate thesurface-relief transmission grating element on a low thermal expansionsubstrate material because the thermal expansion coefficient of thesubstrate material determines how rapidly the grating line spacingchanges as a function of temperature change for surface-relief gratingshaving a grating forming layer thickness that is extremely small incomparison to the substrate thickness. Change in the grating linespacing of a grating element causes a corresponding change in the angleof the beam diffracted by the element, which results in a positionalchange of the focused diffracted beam at the focal plane of theMux/Demux, wavelength channel monitoring or ADWM device incorporatingthe grating element. These changes in focused beam position give rise toincreased optical insertion loss in the device and if large enough causea shifting of data information between adjacent wavelength channels inthe device.

[0191] The change in diffracted beam angle as a function of change ingrating line spacing is calculated by differentiating Equation (1) withrespect to dD, which gives: $\begin{matrix}{{d\quad \theta_{d}} = {- {\frac{\lambda \quad d\quad D}{D^{2}\quad \cos \quad \theta_{d}}.}}} & (7)\end{matrix}$

[0192] When the change in the grating line spacing is due to the thermalexpansion change of the grating substrate material,

dD=αDdT,  (8)

[0193] where α is the thermal expansion coefficient of the substratematerial and dT is the temperature change. Substituting Equation (8)into Equation (7) gives: $\begin{matrix}{{d\quad \theta_{d}} = {{- \frac{\lambda}{D}}{\frac{\alpha \quad d\quad T}{\cos \quad \theta_{d}}.}}} & (9)\end{matrix}$

[0194] Equation (9) is used to calculate how the diffracted beam anglechanges as a function of the thermal expansion coefficient of thesubstrate material used for fabricating photoresist surface-relieftransmission grating elements. For these calculations it was assumedthat the grating element has a λ/D ratio of 1.1 for a wavelength of 1550nanometers and that θ_(i)=θ_(d)=33.4° for a wavelength of 1550nanometers. Using these assumptions, the change in diffracted beam angleassociated with a 70 degree centigrade temperature change and thecorresponding spatial positional change at the focal plane caused bythis angular change was calculated for a Mux/Demux device incorporatinga focusing lens assembly having a focal length of f₁=40 millimeters andfor a wavelength channel monitoring device incorporating a focusing lensassembly having a focal length of f₂=80 millimeters. Results for thesecalculations are presented in Table I for different transmission glasstypes. TABLE I Thermal Change in Change in Spatial Expansion DiffractionPosition at Focal Plane Coefficient Angle in Arc in microns for GlassType (X10⁻⁷/° C.) Seconds f₁ f₂ BK7 70 133.18 25.8 51.6 Fused Silica 5.510.46 2.0 4.0 O'Hara Clear- 0.8 1.52 0.30 0.60 ceram-Z ULE 0.15 0.280.06 0.12

[0195] Assuming that the input/output fiber-optic array in the Mux/Demuxdevice has a 25 micron spacing between fiber centers and that thephotodetector linear array used in the wavelength channel monitoringdevice has a 50 micron spacing between photodetector elements, then itis evident from the data presented in Table I that BK7 should not beused as the substrate material for transmission gratings used in thesedevices unless other means are provided to compensate for the change indiffracted beam angle that occurs with this material as a result oftemperature change. It is also evident from the data in Table I thatfused silica could be used as the substrate material for thetransmission gratings used in these devices but better results would beachieved by using either the O'Hare Clearceram-Z material or the ULEglass material.

[0196] While the device examples presented in this specification havefocused on different arrangements for using surface-relief transmissiongrating elements for fabricating Mux/Demux, on-line wavelength channelmonitoring and add/drop devices used in WDM fiber-optic systems, it isevident that these transmission gratings and the different usagearrangements described in this specification can also be used toconstruct tunable laser sources used in fiber-optic communicationsystems and to build spectrophotometer instruments used by field andlaboratory personnel for measuring the wavelength component propertiesof WDM fiber-optic systems.

[0197] The following examples for the life test results achieved forphotoresist surface-relief transmission grating elements are presentedto illustrate the claimed invention and are not to be deemed limitativethereof Unless otherwise specified, in all examples, all parts are byweight and all temperatures are in degrees centigrade.

[0198] In these life test examples, reference is made to the BellcoreGR1209 and GR1221 reliability guidelines for fiber-optic devices, whichrequire that statistical data be provided based on testing multiplenumbers of the same device and that test data be provided for up to 2000hours of test time. The Bellcore tests require that the item tested,when subject to 85 degrees centigrade and 85% relative humidity, hasless than 0.5 decibel optical insertion loss variation after beingtested for 500 hours at these conditions.

[0199] A change of 0.1 decibel in optical insertion-loss corresponds toa change of 2.276 percent in the radiometric throughput efficiency ofthe item while a 0.5 decibel change corresponds to a 10.875 percentchange in the radiometric throughput efficiency of the item. A change inoptical insertion-loss for the tested surface-relief grating elementscorrelates to a change in the diffraction efficiency of the gratingelement caused by a change in the depth of the surface-relief gratinggroove height h. A change in the grating groove height h can occur inphotoresist surface-relief gratings under conditions that cause thephotoresist to flow.

[0200] All of the grating elements used in the life test experiments forthe examples presented in this specification had a surface-relieftransmission grating fabricated using in Shipley S1813 Photo Resist. Allof the example grating elements used in these life test experiments hada grating line spacing equal to the 633 nanometer wavelength light usedto measure the diffraction efficiency performance of the examples underlife test. The diffraction efficiency for each example grating elementwas performed using the S polarized optical component at the Littrowdiffraction condition, that is, θ_(i)=θ_(d), which for the gratings inthis test corresponded to θ_(i)=θ_(d)=30 degrees. Diffraction efficiencydata for each example grating element was taken before the element wastested at the Bellcore test conditions of 85 degrees centigrade and 85percent relative humidity, and at different time intervals after theelement began testing at these Bellcore test conditions. The number oftime intervals that each element was tested depended on how long it tookthe element to fail, if it failed, or the number of hours the elementhas undergone testing when the data was assembled for the examplesreported in this specification.

[0201] All of the surface-relief transmission diffraction gratingexamples for these life test experiments consisted of circular disksubstrates having a diameter of either 100 or 120 millimeters. Each disksubstrate contained 5 essentially identical plane diffraction gratingsegments arranged symmetrically around the center of the disk. This typeof circular disk grating element is commercially available from HolotekLLC and is sold as a hologon element. Each of the example hologon disksused for these life test experiments was prepared in accordance with theprocessing procedures described in FIG. 3 of this specification, exceptfor the Comparative Examples 1, 2, and 3. An area of one of the 5grating segments on each example hologon disk was circled with apermanent marker pen, the circled area being used to measure thediffraction efficiency of that example before and during testing at theBellcore conditions.

[0202] Each of the example hologon disks for these life test experimentswas configured as either a bare hologon substrate disk that wasuncovered during the life testing by any protective element and hologondisks having a cover glass disk that protected the grating surface,similar to the encapsulated grating surface configuration in FIG. 9 ofthis specification. Different sealing compounds were used, in thedifferent comparative examples that used this covered hologon diskstructure, to bond the cover glass disk to the hologon substrate disk.All covered hologon disk examples in these tests used 0.002 inch shimsto space the hologon substrate grating surface from the cover disksurface.

COMPARATIVE EXAMPLE 1

[0203] The hologon disks for Example 1 were processed using steps 50through 60 of FIG. 3 of this specification, but did not include steps 62and 64 of FIG. 3. After step 60 of FIG. 3, the dried hologon substratedisks and their cleaned matching cover disks were placed in an 82degrees centigrade oven for 1 hour. After cooling down to roomtemperature the hologon substrate disks and their cover disks werebonded together using Norland 61 and 68 ultraviolet light curableadhesives. The Norland 61 adhesive was used to seal the center of thehologon assembly which had a through hole, while the Norland 68 adhesivewas used to seal the circumference of the hologon disk assembly. TheNorland adhesives were cured using a lamp having a ultraviolet spectrumoutput in the range of 350 to 430 nanometers. As part of the initialsealing process, small vent holes were left in the outer sealing ring onthe hologon covered assemblies, and the vented assemblies were placed ina vacuum oven at 70 degrees centigrade for 30 minutes to help remove anyresiduals associated with the ultraviolet adhesives. After removal fromthe vacuum oven the hologons were cooled to room temperature and thevent holes sealed.

[0204] The Norland Optical Adhesive 61, and the Norland Optical Adhesive68, were obtained from Norland Products Incorporated of 695 Joyce KilmerAvenue, New Brunswick, N.J. According to the Material Safety Data Sheetsfor these products, “The specific chemical identity and concentration isbeing withheld from this data sheet as a trade secret.”

[0205] The covered hologon assemblies bonded using the Norland adhesiveswere subjected to a temperature of 85 degrees centigrade and a relativehumidity of 85 percent. In less than 2 hours under these testconditions, both sets of hologons failed completely, that is, thegrating surface structure disappeared under these conditions.

COMPARATIVE EXAMPLE 2

[0206] The uncovered hologon disk for Example 2 was made substantiallyin accordance with the procedure used to prepare the Example 1 hologonswith the exception that the bare photoresist surface of the hologonsubstrate disk, after cooling to room temperature following the 1 hourbake at 82 degrees centigrade, was exposed to a lamp for 30 minuteshaving an ultraviolet spectrum output in the range of 350 to 430nanometers. The grating surface structure of the uncovered hologon madein accordance with this procedure failed completely in less than twohours when subjected to a temperature of 85 degrees centigrade and arelative humidity of 85 percent.

COMPARATIVE EXAMPLE 3

[0207] The uncovered hologon disk for this example was processed usingsteps 50 through 60 of FIG. 3 of this specification, but did not includesteps 62 and 64 of FIG. 3. After step 60 of FIG. 3, the dried barephotoresist surface of the hologon disk was simultaneously subjected toa temperature of about 110 degrees centigrade while being irradiatedwith ultraviolet light at a wavelength of 260 nanometers forapproximately 20 minutes. It was observed that this post-baked/UVexposure procedure caused the hologon to change from a yellow to a clearto a light-to-medium brown color. This uncovered bare hologonexperienced essentially no change in diffraction grating efficiencyafter being tested for over 300 hours at the 85 degrees centigrade and85 percent relative humidity test conditions.

COMPARATIVE EXAMPLE 4

[0208] The uncovered hologon disk for this example was processed usingall of the steps of FIG. 3 of this specification. Exposure of the barephotoresist surface of the hologon disk at ambient room temperature andpressure conditions to the 260 nanometer DUV light source in step 62 ofFIG. 3 bleached the photoresist layer and, thereby changed the color ofthe photoresist layer from a yellow color to an substantially opticallyclear color having no visible observable color tint. The photoresistlayer stays substantially optically clear not only after being heated inprocessing step 64 of FIG. 3 but also after being tested for about 1,000hours at the aforementioned Bellcore test conditions. After exposure tothe DUV light source, the hologon disk was in step 62 of FIG. 3 heatedto a temperature of about 110 to 115 degrees centigrade forapproximately 30 minutes. The uncovered bare hologon made in accordancewith this procedure experienced less than a 0.2 decibel change in itsdiffraction efficiency performance after being subject to testing at atemperature of 85 degrees centigrade and a relative humidity of 85percent for in excess of 1,700 hours and, therefore, meets theaforementioned Bellcore test conditions.

COMPARATIVE EXAMPLE 5

[0209] The uncovered hologon disk for this example was processed usingthe procedure used to prepare Example 4, with the exception that afterthe bare photoresist surface of the hologon is exposed to the 260nanometer radiation for about 10 minutes at ambient room temperature andpressure conditions, the hologon was heated in step 62 of FIG. 3 of thisspecification to a temperature of about 140 to 150 degrees centigradefor about 30 minutes. As was the case for the procedure used to processExample 4, the hologon for these processing conditions was substantiallyoptically clear with no visible observable color tint. The uncoveredbare hologon made in accordance with this procedure experienced lessthan 0.3 decibel change in its diffraction efficiency performance afterbeing subjected to testing at a temperature of 85 degrees centigrade anda relative humidity of 85 percent for in excess of 1,700 hours and,therefore, meets the aforementioned Bellcore test conditions.

COMPARATIVE EXAMPLE 6

[0210] The hologon substrate disk used for this covered hologon diskexample was processed using all of the steps of FIG. 3 of thisspecification, similar to the procedures used to process Examples 4 and5. The processing of this hologon substrate differs from the procedureused to process Examples 4 and 5 in that in 64 of FIG. 3 the hologonsubstrate disk was heated to a temperature of about 130 to 135 degreescentigrade for about 30 minutes. As was the case for the procedures usedto process Examples 4 and 5, the hologon for these processing conditionswas substantially optically clear with no visible observable color tint.

[0211] The hologon substrate for this example was bonded to its cleancover disk using AB 9001 MT Epoxy, manufactured by Fiber Optic Center,Inc. of 23 Centre Street, New Bedford, Mass. The photoresist was removedfrom the hologon substrate disk in the areas where the epoxy adhesivewas to be applied to form the bond. After applying the epoxy to theexample, it sat at room temperature for 15 hours and then wasfinish-cured in a dry oven at 85 degrees centigrade for 30 minutes,followed by 100 degrees centigrade for 30 minutes.

[0212] The bonded hologon of this example, after being subjected to atemperature of 85 degrees centigrade and a relative humidity of 85percent for more than 1,000 hours, had an optical insertion loss of lessthan 0.3 decibels and, therefore, meets the aforementioned Bellcore testconditions.

COMPARATIVE EXAMPLE 7

[0213] The hologon substrate disk used for this covered disk example wasprocessed using the procedure of Example 6. The hologon substrate forthis example was bonded to its clean cover disk using EPO-TEK 353NDepoxy, manufactured by Epoxy Technology of 14 Fortune Drive, Billerica,Mass.; this adhesive is a two component, 100% solids heat curing epoxydesigned for high temperature applications; it is comprised of anacrylonitrile curing agent, and it is also comprised of less than about75 weight percent of Bisphenol F.

[0214] The EPO-TEK 353ND epoxy had a low viscosity, and thus it wasmixed by volume in a ratio of approximately 1 part epoxy to 9 parts offused silica particles sintered together in chain-like formations andsold under the name of “Cab-O-Sil” by the Cabot Corporation of 1020 WestPark Avenue, Kokoma, Ind.

[0215] The photoresist was removed from the hologon substrate disk inthe areas where the epoxy adhesive was to be applied to form the bond.After applying the adhesive to the example, it sat at room temperaturefor 15 hours and then was finish-cured in a dry oven at 85 degreescentigrade for 30 minutes, followed by 100 degrees centigrade for 30minutes.

[0216] The bonded sealed hologon of this example, after being subjectedto a temperature of 85 degrees centigrade and a relative humidity of 85percent for more than 1,000 hours, had an optical insertion loss of lessthan 0.3 decibels and, therefore, meets the aforementioned Bellcore testconditions.

COMPARATIVE EXAMPLE 8

[0217] The hologon substrate disk used for this covered disk example wasprocessed using the procedure for Example 6. The hologon substrate forthis example was bonded to its clean cover disk using Devcon AluminumPutty (F) 10610 Epoxy manufactured by ITW Devon of Danvers, Mass..

[0218] The photoresist was removed from the hologon substrate disk inthe areas where the epoxy adhesive was to be applied to form the bond.After applying the adhesive to the example, it sat at room temperaturefor 15 hours and then was finish-cured in a dry oven at 85 degreescentigrade for 30 minutes, followed by 100 degrees centigrade for 30minutes.

[0219] The bonded hologon of this example, after being subjected to atemperature of 85 degrees centigrade and a relative humidity of 85percent for 500 hours, had an optical insertion loss of approximately0.5 decibels, and after 1,000 hours the optical insertion loss increasedto approximately 1.0 decibels. This example just meets theaforementioned Bellcore test conditions of having no more than a 0.5decibel change in optical insertion loss for 500 hours of test time.

COMPARATIVE EXAMPLE 9

[0220] The hologon substrate disk used for this covered disk example wasprocessed using he procedure of Example 6. The hologon substrate forthis example was bonded to its clean cover disk using Scotch-Weld EpoxyAdhesive DP-190, which contained epoxy resin, kaolin, aliphatic polymerdiamine, and carbon black; this multi-component adhesive was sold by theMinnesota Mining and Manufacturing Corporation of St. Paul, Minn.

[0221] The photoresist was removed from the hologon substrate disk inthe areas where the epoxy adhesive was to be applied to form the bond.After applying the adhesive to the example, it sat at room temperaturefor 15 hours and then was finish-cured in a dry oven at 85 degreescentigrade for 30 minutes, followed by 100 degrees centigrade for 30minutes.

[0222] The bonded sealed hologon of this example, after being subjectedto a temperature of 85 degrees centigrade and a relative humidity of 85percent for more 500 hours, had an optical insertion loss ofapproximately 1.0 decibels, and after 1,000 hours the optical insertionloss increased to approximately 1.6 decibels. This example does not meetthe aforementioned Bellcore test conditions.

COMPARATIVE EXAMPLE 10

[0223] The hologon substrate disk used for this covered disk example wasprocessed using the procedure of Example 6. The hologon substrate forthis example was bonded to its clean cover disk using the same Norland61 and 68 optical adhesives used to bond the hologon disks in Example 1.The bonding technique used for this example is the same as used forExample 1, in which the adhesives are applied to the hologon substratedisk without removing the photoresist form areas where the adhesiveswere applied to from the bond.

[0224] The bonded sealed hologon of this example, after being subjectedto a temperature of 85 degrees centigrade and a relative humidity of 85percent for more 500 hours, had an optical insertion loss in the rangeof 3.3 decibels, and after about 900 hours the optical insertion lossincreased to about 20 decibels. This example does not meet theaforementioned Bellcore test conditions.

COMPARATIVE EXAMPLE 11

[0225] The hologon substrate disk used for this covered disk example wasprocessed using the procedure of Example 6. The hologon substrate forthis example was bonded to its clean cover disk using ELC 2728ultraviolet light curable epoxy manufactured by Electro-Lite Corporationof 43 Miry Brook Road, Danbury, Conn.

[0226] The ultraviolet curable epoxy was applied to the hologonsubstrate disk without removing the photoresist from areas where theepoxy was applied to from the bond. After the epoxy was applied to theexample, it was cured using a lamp having an ultraviolet spectrum outputin the range of about 350 to 430 nanometers.

[0227] The bonded sealed hologon of this example, after being subject toa temperature of 85 degrees centigrade and a relative humidity of 85percent for about 500 hours, had an optical insertion loss in the rangeof 4.7 decibels and after about 900 hours the optical insertion lossincreased to about 5.3 decibels. This example does not meet theaforementioned Bellcore test conditions.

[0228] It is to be understood that the aforementioned description isillustrative only and that changes can be made in the apparatus, in theingredients and their proportions, and in the sequence of combinationsand process steps, as well as in other aspects of the inventiondiscussed herein, without departing from the scope of the invention asdefined in the following claims.

I claim:
 1. An optical wavelength selection apparatus comprising asurface-relief transmission diffraction grating assembly, a collimatinglens assembly for collimating optical beams to said diffraction grating,and a focusing lens assembly for focusing beams diffracted from saiddiffraction grating, wherein said surface relief diffraction gratingassembly is comprised of a substrate and, bonded to said substrate, asurface-relief transmission diffraction grating, and wherein: (a) saidsubstrate is optically homogeneous, consists essentially of materialwhich has a coefficient of thermal expansion of from about 2×10⁻⁵ toabout −1×10⁻⁶ per degree centigrade, has a refractive index of fromabout 1.4 to about 4.0, and has a transmittance of at least about 70percent; (b) said surface-relief transmission diffraction grating iscomprised of a grating forming layer and a surface-relief grating formedin said grating forming layer, wherein:
 1. said surface-relief gratinghas a peak-to-trough height of from about 0.5 to about 5.0 microns, and2. the grating aspect ratio of said peak-to-trough height of saidsurface-relief grating to said grating line spacing of saidsurface-relief grating is from about 0.8 to about 2.0; (c) when theincident optical beam to said surface-relief transmission diffractiongrating assembly and a first order diffracted optical beam from saidsurface-relief transmission diffraction grating assembly havesubstantially equal angles with respect to the normal to the gratingsurface of said surface-relief transmission diffraction gratingassembly, and
 1. when the ratio of the wavelength of the incidentoptical beam to the grating line spacing is from about 0.8 to about 2.0,said surface-relief transmission diffraction grating assembly has afirst diffraction efficiency of at least about 70 percent for the Spolarized optical component of said incident optical beam, and
 2. whenthe ratio of the wavelength of the incident optical beam to the gratingline spacing is from about 0.8 to about 1.2, said surface-relieftransmission diffraction grating assembly has a second diffractionefficiency of at least about 70 percent for the P polarized opticalcomponent of said incident optical beam and said second diffractionefficiency has a value that decreases from at least 70 percent when theratio of the wavelength of the incident optical beam to grating linespacing is about 1.2 to a value of less than 10 percent when the ratioof the wavelength of the incident optical beam to grating line spacingis from about 1.43 to about 2.0; (d) after said surface-relieftransmission diffraction grating assembly has been subjected to atemperature of 85 degrees centigrade and a relative humidity of 85percent for 1,000 hours and then tested in accordance with the procedurespecified in paragraph (c) to determine a third diffraction efficiencyfor the S polarized optical component and a fourth diffractionefficiency for the P polarized optical component, said third diffractionefficiency is less than 6 percent different than said first diffractionefficiency, said fourth diffraction efficiency is less than 6 percentdifferent than said second diffraction efficiency; (e) saidsurface-relief transmission grating assembly is substantially opticallyclear; and (f) after said surface-relief transmission grating assemblyhas been subjected to a temperature of 85 degrees centigrade and arelative humidity of 85 percent for 1,000 hours, said surface-relieftransmission grating assembly is still substantially optically clear. 2.The optical wavelength selection apparatus as recited in claim 1,wherein said surface-relief grating has a substantially sinusoidalshape.
 3. The optical wavelength selection apparatus as recited in claim1, wherein said surface-relief grating has a substantially rectangularshape.
 4. The optical wavelength selection apparatus as recited in claim1, wherein said surface-relief grating has a substantially triangularshape.
 5. The optical wavelength selection apparatus as recited in claim1, wherein said first diffraction efficiency and said second diffractionefficiency are no greater than about 10 percent different from eachother when the ratio of the wavelength of the incident beam to gratingline spacing is from about 0.8 to 1.2.
 6. The optical wavelengthselection apparatus as recited in claim 1, wherein said substrateintroduces less than 0.25 wave of either spherical or cylindricalwavefront power into the beam transmitted from said surface-relieftransmission diffraction grating assembly.
 7. The optical wavelengthselection apparatus as recited in claim 1, wherein said substrate iscomprised of a first flat surface and a second flat surface.
 8. Theoptical wavelength selection apparatus as recited in claim 7, whereinsaid first flat surface and said second flat surface are substantiallyparallel to each other.
 9. The optical wavelength selection apparatus asrecited in claim 1, wherein said substrate has a coefficient of thermalexpansion from about 6×10⁻⁷ to about −6×10⁻⁷ per degree centigrade. 10.The optical wavelength selection apparatus as recited in claim 1,wherein said substrate is comprised of fused silica doped with titanium.11. The optical wavelength selection apparatus as recited in claim 1,wherein said substrate has a refractive index of from about 1.43 toabout 1.7.
 12. The surface-relief transmission grating assembly asrecited in claim 1, wherein said grating forming layer consistsessentially of material with an index of refraction of from about 1.4 toabout 1.8.
 13. The surface-relief transmission grating assembly asrecited in claim 1, wherein said grating forming layer consistsessentially of material with an index of refraction of from about 1.43to about 1.55.
 14. The optical wavelength selection apparatus as recitedin claim 1, wherein said grating forming layer is formed from aphotoresist material.
 15. The optical wavelength selection apparatus asrecited in claim 14, wherein said photoresist material is a positivephotoresist material.
 16. The optical wavelength selection apparatus asrecited in claim 15, wherein said positive photoresist material is adiazonaphthoquinone-novolak positive photoresist material.
 17. Theoptical wavelength selection apparatus as recited in claim 1, whereinsaid surface-relief grating has a grating line spacing of from about1.11 to about 2 microns.
 18. The optical wavelength selection apparatusas recited in claim 1, wherein said surface-relief transmissiondiffraction grating is a plane diffraction grating having parallel,equidistantly spaced grating lines which reside on a flat surface. 19.The optical wavelength selection apparatus as recited in claim 1,wherein said surface-relief grating has a grating aspect ratio of fromabout 1.3 to about 2.0.
 20. The optical wavelength selection apparatusas recited in claim 7, wherein said first flat surface and said secondflat surface are substantially non-parallel to each other.
 21. Theoptical wavelength selection apparatus as recited in claim 14, whereinsaid surface-relief grating is formed in the photoresist material usingan optical interference pattern having a period equal to the gratingline spacing.
 22. The optical wavelength selection apparatus as recitedin claim 1, wherein said surface-relief grating is formed in the gratingforming layer by replication means.
 23. The optical wavelength selectionapparatus as recited in claim 1, wherein said surface-relieftransmission diffraction grating assembly is optically bonded to a glassblock element having at least one flat surface.
 24. The opticalwavelength selection apparatus as recited in claim 1, wherein saidsurface-relief transmission diffraction grating assembly is attached toa glass block element having at least one flat surface with anencapsulated air layer existing between said surface-relief grating ofsaid surface-relief transmission diffraction grating assembly and saidflat surface of said glass block element to which said surface-relieftransmission diffraction grating assembly is attached.
 25. The opticalwavelength selection apparatus as recited in claim 1, wherein saidsubstrate has a transmittance of at least about 70 percent for thewavelength spectrum of from about 1280 to about 1620 nanometers.
 26. Theoptical wavelength selection apparatus as recited in claim 5, whereinsaid first and second diffraction efficiencies are no greater than about5 percent different from each other when said surface-relieftransmission diffraction grating assembly is used with a wavelengthspectrum of from about 1280 to about
 1620. 27. The optical wavelengthselection apparatus as recited in claim 1, wherein said opticalwavelength selection apparatus further comprises a second surface-relieftransmission diffraction grating positioned after said surface-relieftransmission diffraction grating.
 28. The optical wavelength selectionapparatus recited in claim 27, wherein said surface-relief transmissiondiffraction grating and said second surface-relief transmissiondiffraction grating are incorporated in a glass block element.
 29. Theoptical wavelength selection apparatus as recited in claim 1, whereinsaid optical wavelength selection apparatus further comprises a beamfolding mirror element.
 30. The optical wavelength selection apparatusas recited in claim 1, wherein optical wavelength selection apparatusfurther comprises a mirror element positioned after said surface-relieftransmission diffraction grating and orientated to reflect an opticalbeam diffracted from said surface-relief transmission diffractiongrating back to said surface-relief transmission diffraction grating.31. The optical wavelength selection apparatus as recited in claim 30,wherein said surface-relief transmission diffraction grating and saidmirror element are incorporated in a glass block element.
 32. Theoptical wavelength selection apparatus as recited in claim 1, whereinsaid optical wavelength selection apparatus further comprises a beamexpansion prism element.
 33. The optical wavelength selection apparatusas recited in claim 1, wherein said optical wavelength selectionapparatus further comprises a linearizing prism element.
 34. The opticalwavelength selection apparatus as recited in claim 1, wherein saidoptical wavelength selection apparatus further comprises asurface-relief reflection grating element positioned after saidsurface-relief transmission diffraction grating and orientated todiffract an optical beam diffracted from said surface-relieftransmission diffraction grating ba ck to said surface-relieftransmission diffraction grating.
 35. The optical wavelength selectionapparatus as recited in claim 34, wherein said surface-relieftransmission diffraction grating and said surface-relief reflectiongrating element are incorporated in a glass block element.
 36. Theoptical wavelength selection apparatus as recited in claim 28, whereinsaid optical wavelength selection apparatus further comprises a mirrorelement positioned after said glass block element and orientated toreflect an optical beam diffracted from said glass block element back tosaid glass block element.
 37. The optical wavelength selection apparatusas recited in claim 28, wherein said optical wavelength selectionapparatus further comprises a surface-relief reflection grating elementpositioned after said glass block element and orientated to diffract anoptical beam diffracted from said glass block element back to said glassblock element.
 38. The optical wavelength selection apparatus as recitedin claim 28, wherein said optical wavelength selection apparatus furthercomprises a third surface-relief transmission diffraction gratingpositioned after said glass block element.
 39. The optical wavelengthselection apparatus as recited in claim 1, wherein said opticalwavelength selection apparatus further comprises a second surface-relieftransmission diffraction grating disposed between said surface-relieftransmission diffraction grating and said focusing lens assembly. 40.The optical wavelength selection apparatus as recited in claim 39,wherein said surface-relief transmission diffraction grating and saidsecond surface-relief transmission diffraction grating are incorporatedinto a glass block element.
 41. The optical wavelength selectionapparatus as recited in claim 1, further comprising: (a) a birefringentcrystal element positioned before said surface-relief transmissiondiffraction grating for splitting said S polarized optical componentfrom said P polarized optical component for optical beams incident tosaid surface-relief transmission diffraction grating and for combiningsaid S polarized optical component with said P polarized opticalcomponent for optical beams diffracted from said surface-relieftransmission diffraction grating, (b) a halfwave retardation platepositioned before said surface-relief transmission diffraction gratingfor converting said P polarized optical component incident to saidsurface-relief transmission diffraction grating to a S polarized opticalcomponent that is incident on said surface-relief transmissiondiffraction grating and for converting said S polarized opticalcomponent diffracted from said surface-relief transmission grating to aP polarized optical component, and (c) a dihedral mirror elementpositioned after said surface-relief transmission diffraction gratingfor retroreflecting a first diffracted optical beam and a seconddiffracted optical beam from said surface-relief transmissiondiffraction grating back to said surface-relief transmission diffractiongrating.
 42. An optical wavelength selection apparatus comprising asurface-relief transmission diffraction grating assembly, a collimatinglens assembly for collimating optical beams to said diffraction grating,and a focusing lens assembly for focusing beams diffracted from saiddiffraction grating, wherein said surface relief diffraction gratingassembly is comprised of a substrate and, bonded to said substrate, asurface-relief transmission diffraction grating, and wherein: (a) saidsubstrate is optically homogeneous, consists essentially of materialwhich has a coefficient of thermal expansion of from about 2×10⁻⁵ toabout −1×10⁻⁶ per degree centigrade, has a refractive index of fromabout 1.4 to about 4.0, and has a transmittance of at least about 70percent; (b) said surface-relief transmission diffraction grating iscomprised of a grating forming layer and a surface-relief grating formedin said grating forming layer, wherein:
 1. said surface-relief gratinghas a peak-to-trough height of from about 0.5 to about 5.0 microns, and2. the grating aspect ratio of said peak-to-trough height of saidsurface-relief grating to said grating line spacing of saidsurface-relief grating is from about 0.8 to about 2.0; (c) when theincident optical beam to said surface-relief transmission diffractiongrating assembly and a first order diffracted optical beam from saidsurface-relief transmission diffraction grating assembly havesubstantially equal angles with respect to the normal to the gratingsurface of said surface-relief transmission diffraction gratingassembly, and
 1. when the ratio of the wavelength of the incidentoptical beam to the grating line spacing is from about 0.8 to about 2.0,said surface-relief transmission diffraction grating assembly has afirst diffraction efficiency of at least about 70 percent for the Spolarized optical component of said incident optical beam, and
 2. whenthe ratio of the wavelength of the incident optical beam to the gratingline spacing is from about 0.8 to about 1.2, said surface-relieftransmission diffraction grating assembly has a second diffractionefficiency of at least about 70 percent for the P polarized opticalcomponent of said incident optical beam and said second diffractionefficiency has a value that decreases from at least 70 percent when theratio of the wavelength of the incident optical beam to grating linespacing is about 1.2 to a value of less than 10 percent when the ratioof the wavelength of the incident optical beam to grating line spacingis from about 1.43 to about 2.0; and (d) after said surface-relieftransmission diffraction grating assembly has been subjected to atemperature of 85 degrees centigrade and a relative humidity of 85percent for 1,000 hours and then tested in accordance with the procedurespecified in paragraph (c) to determine a third diffraction efficiencyfor the S polarized optical component and a fourth diffractionefficiency for the P polarized optical component, said third diffractionefficiency is less than 6 percent different than said first diffractionefficiency, said fourth diffraction efficiency is less than 6 percentdifferent than said second diffraction efficiency.
 43. The opticalwavelength selection apparatus as recited in claim 42, wherein saidsurface-relief grating has a substantially sinusoidal shape.
 44. Theoptical wavelength selection apparatus as recited in claim 42, whereinsaid surface-relief grating has a substantially rectangular shape. 45.The optical wavelength selection apparatus as recited in claim 42,wherein said surface-relief grating has a substantially triangularshape.
 46. The optical wavelength selection apparatus as recited inclaim 42, wherein said first diffraction efficiency and said seconddiffraction efficiency are no greater than about 10 percent differentfrom each other when the ratio of the wavelength of the incident beam tograting line spacing is from about 0.8 to 1.2.
 47. The opticalwavelength selection apparatus as recited in claim 42, wherein saidsubstrate introduces less than 0.25 wave of either spherical orcylindrical wavefront power into the beam transmitted from saidsurface-relief transmission diffraction grating assembly.
 48. Theoptical wavelength selection apparatus as recited in claim 42, whereinsaid substrate is comprised of a first flat surface and a second flatsurface.
 49. The optical wavelength selection apparatus as recited inclaim 48, wherein said first flat surface and said second flat surfaceare substantially parallel to each other.
 50. The optical wavelengthselection apparatus as recited in claim 42, wherein said substrate has acoefficient of thermal expansion from about 6×10⁻⁷ to about −6×10⁻⁷ perdegree centigrade.
 51. The optical wavelength selection apparatus asrecited in claim 42, wherein said substrate is comprised of fused silicadoped with titanium.
 52. The optical wavelength selection apparatus asrecited in claim 42, wherein said substrate has a refractive index offrom about 1.43 to about 1.7.
 53. The surface-relief transmissiongrating assembly as recited in claim 42, wherein said grating forminglayer consists essentially of material with an index of refraction offrom about 1.4 to about 1.8.
 54. The surface-relief transmission gratingassembly as recited in claim 42, wherein said grating forming layerconsists essentially of material with an index of refraction of fromabout 1.43 to about 1.55.
 55. The optical wavelength selection apparatusas recited in claim 42, wherein said grating forming layer is formedfrom a photoresist material.
 56. The optical wavelength selectionapparatus as recited in claim 55, wherein said photoresist material is apositive photoresist material.
 57. The optical wavelength selectionapparatus as recited in claim 56, wherein said positive photoresistmaterial is a diazonaphthoquinone-novolak positive photoresist material.58. The optical wavelength selection apparatus as recited in claim 42,wherein said surface-relief grating has a grating line spacing of fromabout 1.11 to about 2 microns.
 59. The optical wavelength selectionapparatus as recited in claim 42, wherein said surface-relieftransmission diffraction grating is a plane diffraction grating havingparallel, equidistantly spaced grating lines which reside on a flatsurface.
 60. The optical wavelength selection apparatus as recited inclaim 42, wherein said surface-relief grating has a grating aspect ratioof from about 1.3 to about 2.0.
 61. The optical wavelength selectionapparatus as recited in claim 48, wherein said first flat surface andsaid second flat surface are substantially non-parallel to each other.62. The optical wavelength selection apparatus as recited in claim 55,wherein said surface-relief grating is formed in the photoresistmaterial using an optical interference pattern having a period equal tothe grating line spacing.
 63. The optical wavelength selection apparatusas recited in claim 42, wherein said surface-relief grating is formed inthe grating forming layer by replication means.
 64. The opticalwavelength selection apparatus as recited in claim 42, wherein saidsurface-relief transmission diffraction grating assembly is opticallybonded to a glass block element having at least one flat surface. 65.The optical wavelength selection apparatus as recited in claim 42,wherein said surface-relief transmission diffraction grating assembly isattached to a glass block element having at least one flat surface withan encapsulated air layer existing between said surface-relief gratingof said surface-relief transmission diffraction grating assembly andsaid flat surface of said glass block element to which saidsurface-relief transmission diffraction grating assembly is attached.66. The optical wavelength selection apparatus as recited in claim 42,wherein said substrate has a transmittance of at least about 70 percentfor the wavelength spectrum of from about 1280 to about 1620 nanometers.67. The optical wavelength selection apparatus as recited in claim 46,wherein said first and second diffraction efficiencies are no greaterthan about 5 percent different from each other when said surface-relieftransmission diffraction grating assembly is used with a wavelengthspectrum of from about 1280 to about
 1620. 68. The optical wavelengthselection apparatus as recited in claim 42, wherein said opticalwavelength selection apparatus further comprises a second surface-relieftransmission diffraction grating positioned after said surface-relieftransmission diffraction grating.
 69. The optical wavelength selectionapparatus recited in claim 68, wherein said surface-relief transmissiondiffraction grating and said second surface-relief transmissiondiffraction grating are incorporated in a glass block element.
 70. Theoptical wavelength selection apparatus as recited in claim 42, whereinsaid optical wavelength selection apparatus further comprises a beamfolding mirror element.
 71. The optical wavelength selection apparatusas recited in claim 42, wherein optical wavelength selection apparatusfurther comprises a mirror element positioned after said surface-relieftransmission diffraction grating and orientated to reflect an opticalbeam diffracted from said surface-relief transmission diffractiongrating back to said surface-relief transmission diffraction grating.72. The optical wavelength selection apparatus as recited in claim 71,wherein said surface-relief transmission diffraction grating and saidmirror element are incorporated in a glass block element.
 73. Theoptical wavelength selection apparatus as recited in claim 42, whereinsaid optical wavelength selection apparatus further comprises a beamexpansion prism element.
 74. The optical wavelength selection apparatusas recited in claim 42, wherein said optical wavelength selectionapparatus further comprises a linearizing prism element.
 75. The opticalwavelength selection apparatus as recited in claim 42, wherein saidoptical wavelength selection apparatus further comprises asurface-relief reflection grating element positioned after saidsurface-relief transmission diffraction grating and orientated todiffract an optical beam diffracted from said surface-relieftransmission diffraction grating back to said surface-relieftransmission diffraction grating.
 76. The optical wavelength selectionapparatus as recited in claim 75, wherein said surface-relieftransmission diffraction grating and said surface-relief reflectiongrating element are incorporated in a glass block element.
 77. Theoptical wavelength selection apparatus as recited in claim 69, whereinsaid optical wavelength selection apparatus further comprises a mirrorelement positioned after said glass block element and orientated toreflect an optical beam diffracted from said glass block element back tosaid glass block element.
 78. The optical wavelength selection apparatusas recited in claim 69, wherein said optical wavelength selectionapparatus further comprises a surface-relief reflection grating elementpositioned after said glass block element and orientated to diffract anoptical beam diffracted from said glass block element back to said glassblock element.
 79. The optical wavelength selection apparatus as recitedin claim 69, wherein said optical wavelength selection apparatus furthercomprises a third surface-relief transmission diffraction gratingpositioned after said glass block element.
 80. The optical wavelengthselection apparatus as recited in claim 42, wherein said opticalwavelength selection apparatus further comprises a second surface-relieftransmission diffraction grating disposed between said surface-relieftransmission diffraction grating and said focusing lens assembly. 81.The optical wavelength selection apparatus as recited in claim 80,wherein said surface-relief transmission diffraction grating and saidsecond surface-relief transmission diffraction grating are incorporatedinto a glass block element.
 82. The optical wavelength selectionapparatus as recited in claim 42, further comprising: (a) a birefringentcrystal element positioned before said surface-relief transmissiondiffraction grating for splitting said S polarized optical componentfrom said P polarized optical component for optical beams incident tosaid surface-relief transmission diffraction grating and for combiningsaid S polarized optical component with said P polarized opticalcomponent for optical beams diffracted from said surface-relieftransmission diffraction grating, (b) a halfwave retardation platepositioned before said surface-relief transmission diffraction gratingfor converting said P polarized optical component incident to saidsurface-relief transmission diffraction grating to a S polarized opticalcomponent that is incident on said surface-relief transmissiondiffraction grating and for converting said S polarized opticalcomponent diffracted from said surface-relief transmission grating to aP polarized optical component, and (c) a dihedral mirror elementpositioned after said surface-relief transmission diffraction gratingfor retroreflecting a first diffracted optical beam and a seconddiffracted optical beam from said surface-relief transmissiondiffraction grating back to said surface-relief transmission diffractiongrating.